These are some of the classical sources on the Druids. That is Roman and Greek writers. Each section gives you the book and Volume and page number.
Throughout Gaul there are two classes of persons of definite account and dignity. As for the common folk, they are treated almost as slaves, venturing naught of themselves, never taken into counsel. The more part of them, oppressed as they are either by debt, or by the heavy weight of tribute, or by the wrongdoing of the more powerful men, commit themselves in slavery to the nobles, who have, in fact, the same rights over them as masters over slaves. Of the two classes above-mentioned, one consists of Druids, the other of knights. “The former are concerned with divine worship, the due performance of sacrifices, public and private, and the interpretation of ritual questions: a great number of young men gather about them for the sake of instruction and hold them in great honor. In fact, it is they who decide in almost all disputes, public and private; and if any crime has been committed, or murder done, or there is any dispute about succession or boundaries, they also decide it, determining rewards and penalties: if any person or people does not abide by their decision, they ban such from sacrifice, which is their heaviest penalty. Those that are so banned are reckoned as impious and criminal; all men move out of their path and shun their approach and conversation, for fear they may get some harm from their contact, and no justice is done if they seek it, no distinction falls to their share. “Of all these Druids one is chief, who has the highest authority among them. At his death, either any other that is pre-eminent in position succeeds, or, if there be several of equal standing, they strive for the primacy by the vote of the Druids, or sometimes even with armed force. These Druids, at a certain time of the year, meet within the borders of Carnutes, whose territory is reckoned as the center of all Gaul, and sit in conclave in a consecrated spot. Thither assemble from every side all that have disputes, and they obey the decisions and judgments of the Druids. It is believed that their rule of life was discovered in Britain and transferred thence to Gaul; and to-day those who would study the subject more accurately journey, as a rule, to Britain to learn it.
Cæsar, C. J., “De Bello Gallico”, vi, 13
The Druids usually hold aloof from war, and do not pay war taxes with the rest; they are excused from military service and exempt from all liabilities. Tempted by these great rewards, many young men assemble of their own motion to receive their training; many are sent by parents and relatives. Report says that in the school of the Druids they learn by heart a great number of verses, and therefore some persons remain twenty years under training. And they do not think it proper to commit these utterances to writing, although in almost all other matters, and in their public and private accounts, they make use of Greek letters. I believe they have adopted the practice for two reasons – that they do not wish the rule to become common property, nor those who learn the rule to rely on writing and so neglect the cultivation of the memory; and, in fact, it does usually happen that the assistance of writing tends to relax the diligence of the student and the action of the memory. The cardinal doctrine which they seek to teach is that souls do not die, but after death pass from one to another; and this belief, as the fear of death, is thereby cast aside, they hold to be the greatest incentive to valor. Besides this, they have many discussions touching the stars and their movement, the size of the universe and of the earth, the order of nature, the strength and the powers of the immortal gods, and hand down their lore to the young men.
Cæsar, C. J., “De Bello Gallico”, vi, 14
The whole nation of the Gauls is greatly devoted to ritual observances and for that reason those who are smitten with the more grievous maladies and who are engaged in the perils of battle either sacrifice human victims or vow so to do, employing the Druids as ministers for such sacrifices. They believe, in effect, that, unless for a man’s life a man’s life be paid, the majesty of the immortal gods may not be appeased; and in public, as in private, life they observe an ordinance of sacrifices of the same kind. Others use figures of immense size, whose limbs, woven out of twigs, they fill with living men and set on fire, and the men perish in a sheet of flame. They believe that the execution of those who have been caught in the act of theft or robbery or some crime is more pleasing to the immortal gods; but when the supply of such fails they resort to the execution even of the innocent.
Cæsar, C. J., “De Bello Gallico”, vi, 16
The Gauls affirm that they are all descended from a common father, Dis, and say that this is the tradition of the Druids.
Cæsar, C. J., “De Bello Gallico”, vi, 18, 1,
The Germans differ much from this manner of living. They have no Druids to regulate divine worship, no zeal for sacrifices.
Cæsar, C. J., “De Bello Gallico”, vi, 21, 1
Nor is the practice of divination disregarded even among uncivilized tribes if indeed there are Druids in Gaul – and there are, for I knew one of them myself, Divitiacus, the Æduan, your guest and eulogist. He claimed to have that knowledge of nature which the Greeks call “physiologia”, and he used to make predictions, sometimes by means of augury and sometimes by means of conjecture.
Cicero, “De Divinatione”, I xli, 90,
The Pythagorean doctrine prevails among them [the Gauls], teaching that the souls of men are immortal and live again for a fixed number of years inhabited in another body.
Diodorus Siculus, “Histories”, v, 28, 6
And there are among them [the Gauls] composers of verses whom they call Bards; these singing to instruments similar to a lyre, applaud some, while they vituperate others. “They have philosophers and theologians who are held in much honor and are called Druids; they have sooth-sayers too of great renown who tell the future by watching the flights of birds and by the observation of the entrails of victims; and everyone waits upon their word. When they attempt divination upon important matters they practice a strange and incredible custom, for they kill a man by a knife-stab in the region above the midriff, and after his fall they foretell the future by the convulsions of his limbs and the pouring of his blood, a form of divination in which they have full confidence, as it is of old tradition. “It is a custom of the Gauls that no one performs a sacrifice without the assistance of a philosopher, for they say that offerings to the gods ought only to made through the mediation of these men, who are learned in the divine nature and, so to speak, familiar with it, and it is through their agency that the blessings of the gods should properly be sought. It is not only in times of peace, but in war also, that these seers have authority, and the incantations of the bards have effect on friends and foes alike. Often when the combatants are ranged face to face, and swords are drawn and spears bristling, these men come between the armies and stay the battle, just as wild beasts are sometimes held spellbound. Thus even among the most savage barbarians anger yields to wisdom, and Mars is shamed before the Muses.
Diodorus Siculus, “Histories”, v, 31, 2-5
Some say that the study of philosophy was of barbarian origin. For the Persians had their Magi, the Babylonians or the Assyrians the Chaldeans, the Indians their Gymnosophists, while the Kelts and the Galatæ had seers called Druids and Semnotheoi, or so Aristotle says in the “Magic”, and Sotion in the twenty-third book of his Succession of Philosophers.
Diogenes Laertius, “Vitæ”, intro., i:
Those who think that philosophy is an invention of the barbarians explain the systems prevailing among each people. They say that the Gymnosophists and Druids make their pronouncements by means of riddles and dark sayings, teaching that the gods must be worshipped, and no evil done, and manly behavior maintained.
Diogenes Laertius, “Vitæ”, intro., 5
Among all the Gallic peoples, generally speaking, there are three sets of men who are held in exceptional honor: the Bards, the Vates, and the Druids. The Bards are singers and poets; the Vates, diviners and natural philosophers; while the Druids, in addition to natural philosophy, study also moral philosophy. The Druids are considered the most just of men, and on this account they are entrusted with the decision, not only of the private disputes but of the public disputes as well; so that, in former times, they even arbitrated cases of war and made the opponents stop when they were about to line up for battle, and the murder cases, in particular, had been turned over to them for decision. Further, when there is a big yield [of criminals for sacrifice] from these cases, there is forthcoming a big yield from the land too, as they think. However, not only the Druids, but others as well, say that men’s souls, and also the universe are indestructible, although both fire and water at some time or other prevail over them.
Strabo, “Geographica”, iv, 4, c. 197, 4
But the Romans put a stop to these customs, as well as to all those connected with the sacrifices and divinations that are opposed to our usages. They used to strike a human being, whom they had devoted to death, in the back with a sabre, and then divine from his death-struggle. But they would not sacrifice without the Druids. We are told of still other kinds of human sacrifices; for example, they would shoot victims to death with arrows, or impale them in the temples, or having devised a colossus of straw and wood, throw into the colossus cattle and wild animals of all sorts and human beings, and then make a burnt offering of the whole thing.
Strabo, “Geographica”, iv, 4, c. 198, 5
According to the Druids, a part of the population [of Gaul] was indigenous, but some of the people came from outlying islands and lands beyond the Rhine, driven from their homes by repeated wars and by the inroads of the sea.
Ammianus Marcellinus, xv, 9, 4
In these regions, as the people gradually became civilised, attention to the gentler arts became commoner, a study introduced by the Bards, and the Euhages, and the Druids. It was the custom of the Bards to celebrate the brave deeds of their famous men in epic verse accompanied by the sweet strain of the lyre, while Euhages strove to explain the high mysteries of nature. Between them came the Druids, men of greater talent, members of the intimate fellowship of the Pythagorean faith; they were uplifted by searchings into secret and sublime things, and with grand contempt for mortal lot they professed the immortality of the soul.
Ammianus Marcellinus, xv, 9, 8
He [the Emperor Claudius] very thoroughly suppressed the barbarous and inhuman religion of the Druids in Gaul, which at the time of Augustus had merely been forbidden to Roman citizens.
Suetonius, “Claudius”, 25
There still remain traces of atrocious customs no longer practised, and although they now refrain from outright slaughter, yet they still draw blood from the victims led to the altar. They have, however, their own kind of eloquence, and teachers of wisdom called Druids. They profess to know the size and shape of the world, the movements of the heavens and of the stars, and the will of the gods. They teach many things to the nobles of Gaul in a course of instruction lasting as long as twenty years, meeting in secret either in a cave or secluded dales. One of their dogmas has come to common knowledge, namely, that souls are eternal and that there is another life in the infernal regions, and this has been permitted manifestly because it makes the multitude readier for war. And it is for this reason too that they burn or bury with their dead, things appropriate to them in life, and that in times past they even used to defer the completion of business and the payment of debts until their arrival in another world. Indeed, there were some of them who flung themselves willingly on the funeral pyres of their relatives in order to share the new life with them.
Pomponius Mela, “De Situ Orbis”, iii, 2, 18 and 19
And you, O Druids, now that the clash of battle is stilled, once more have you returned to your barbarous ceremonies and to the savage usage of your holy rites. To you alone it is given to know the truth about the gods and deities of the sky, or else you alone are ignorant of this truth. The innermost groves of far-off forests are your abodes. And it is you who say that the shades of the dead seek not the silent land of Erebus and the pale halls of Pluto; rather, you tell us that the same spirit has a body again elsewhere, and that death, if what you sing is true, is but the midpoint of long life.
Lucan , “Pharsalia”, i, 450-8
Here we must mention the awe felt for this plant by the Gauls. The Druids – for so their magicians are called – held nothing more sacred than the mistletoes and the tree that bears it, always supposing that tree to be the oak. But they choose groves formed of oaks for the sake of the tree alone, and they never perform any of their rites except in the presence of a branch of it; so that it seems probable that the priests themselves may derive their name from the Greek word for that tree. In fact, they think that everything that grows on it has been sent from heaven and is proof that the tree was chosen by the god himself. The mistletoe is found but rarely upon the oak; and when found, is gathered with due religious ceremony, if possible on the sixth day of the moon, (for it is by the moon that they measure their months and years, and also their ages of thirty years). They choose this day because the moon, though not yet in the middle of her course, has already considerable influence. They call the mistletoe by a name meaning, in their language, the all-healing. Having made preparation for sacrifice and a banquet beneath the trees, they bring thither two white bulls, whose horns are bound then for the first time. Clad in a white robe, the priest ascends the tree and cuts the mistletoe with a golden sickle, and it is received by others in a white cloak. Then they kill the victims, praying that god will render this gift of his propitious to those to whom he has granted it. They believe that the mistletoe, taken in drink, imparts fecundity to barren animals, and that it is an antidote for all poisons. Such are the religious feelings that are entertained toward trifling things by many peoples.
Pliny , “Nat. Hist.”, xvi, 249
Similar to savin is the plant called selago. It is gathered without using iron and by passing the right hand through the left sleeve of the tunic, as though in the act of committing a theft. The clothing must be white, the feet washed and bare, and an offering of wine and bread made before the gathering. The Druids of Gaul say that the plant should be carried as a charm against every kind of evil, and that the smoke of it is good for diseases of the eyes. The Druids, also, use a certain marsh-plant that they call samolus, this must be gathered with the left hand, when fasting, and is a charm against the diseases of cattle. But the gatherer must not look behind him, nor lay the plant anywhere except in the drinking-troughs.
Pliny, “Nat. Hist.”, xxiv, 103 – 104
There is also another kind of egg, of much renown in the Gallic provinces, but ignored by the Greeks. In the summer, numberless snakes entwine themselves into a ball, held together by a secretion from their bodies and by their spittle. This is called anguinum. The Druids say that hissing serpents throw this up into the air, and that it must be caught in a cloak, and not allowed to touch the ground; and that one must instantly take to flight on horseback, as the serpents will pursue until some stream cuts them off. It may be tested, they say, by seeing if it floats against the current of a river, even though it be set in gold. But as it is the way of magicians to cast a veil about their frauds, they pretend that these eggs can only be taken on a certain day of the moon, as though it rested with mankind to make the moon and the serpents accord as to the moment of the operation. I myself, however, have seen one of these eggs, it was round, and about as large as a smallish apple; the shell was cartilaginous, and pocked like the arms of a polypus. The druids esteem it highly. It is said to ensure success in law-suits and a favourable reception with princes; but this is false, because a man of the Vocontii, who was also a Roman knight, kept one of these eggs in his bosom during a trial, and was put to death by the Emperor Claudius, as far as I can see, for that reason alone.
Pliny, “Nat. Hist.”, xxix, 52
It [magic] flourished in the Gallic provinces, too, even down to a period within our memory; for it was in the time of the Emperor Tiberius that a decree was issued against their Druids and the whole tribe of diviners and physicians. But why mention all this about a practice that has even crossed the ocean and penetrated to the utmost parts of the earth? At the present day, Britannia is still fascinated by magic, and performs its rites with so much ceremony that it almost seems as though it was she who had imparted the cult to the Persians. To such a degree do peoples throughout the whole world, although unlike and quite unknown to one another, agree upon this one point. Therefore we cannot too highly appreciate our debt to the Romans for having put an end to this monstrous cult, whereby to murder a man was an act of the greatest devoutness, and to eat his flesh most beneficial.
Pliny, “Nat. Hist.”, xxx, 13
On the shore stood the opposing army with its dense array of armed warriors, while between the ranks dashed women in black attire like the Furies, with hair dishevelled, waving brands. All around, the Druids, lifting up their hands to heaven and pouring forth dreadful imprecations, scared our soldiers by the unfamiliar sight, so that, as if their limbs were paralysed, they stood motionless and exposed to wounds. Then urged by general’s appeal and mutual encouragements not to quail before a troop of frenzied women, they bore the standard onwards, smote down all resistance, and wrapped the foe in the flames of his own brands. A force was next set over the conquered, and their groves, devoted to inhuman superstitions, were destroyed. They deemed it, indeed, a duty to cover their altars with the blood of captives and to consult their deities through human entrails.
Tacitus, “Annals”, xiv, 30
The Gauls, they remembered, had captured the city in former days, but, as the abode of Jupiter was uninjured, the Empire had survived; whereas now the Druids declared, with the prophetic utterances of an idle superstition, and this fatal conflagration [of the Capitol] was a sign of the anger of heaven, and portended universal empire for the Transalpine nations.