As part of his discussion in On Liberty of how freedom of speech helps avoid the dangers of one-sided thought, John Stuart Mill makes the case for Greek and Roman thought as an important part of our moral and ethical heritage. In Chapter 2 “Of the Liberty of Thought and Discussion,” paragraphs 37 and 38, he writes:

It may be objected, “But some received principles, especially on the highest and most vital subjects, are more than half-truths. The Christian morality, for instance, is the whole truth on that subject, and if any one teaches a morality which varies from it, he is wholly in error.” … But before pronouncing what Christian morality is or is not, it would be desirable to decide what is meant by Christian morality. If it means the morality of the New Testament, I wonder that any one who derives his knowledge of this from the book itself, can suppose that it was announced, or intended, as a complete doctrine of morals. The Gospel always refers to a pre-existing morality, and confines its precepts to the particulars in which that morality was to be corrected, or superseded by a wider and higher… St. Paul, a declared enemy to this Judaical mode of interpreting the doctrine and filling up the scheme of his Master, equally assumes a pre-existing morality, namely, that of the Greeks and Romans; and his advice to Christians is in a great measure a system of accommodation to that; even to the extent of giving an apparent sanction to slavery. What is called Christian, but should rather be termed theological, morality, was not the work of Christ or the Apostles, but is of much later origin… That mankind owe a great debt to this morality, and to its early teachers, I should be the last person to deny; but I do not scruple to say of it, that it is, in many important points, incomplete and one-sided, and that unless ideas and feelings, not sanctioned by it, had contributed to the formation of European life and character, human affairs would have been in a worse condition than they now are. Christian morality (so called) has all the characters of a reaction; it is, in great part, a protest against Paganism. Its ideal is negative rather than positive; passive rather than active; Innocence rather than Nobleness; Abstinence from Evil, rather than energetic Pursuit of Good: in its precepts (as has been well said) “thou shalt not” predominates unduly over “thou shalt.” In its horror of sensuality, it made an idol of asceticism, which has been gradually compromised away into one of legality. It holds out the hope of heaven and the threat of hell, as the appointed and appropriate motives to a virtuous life: in this falling far below the best of the ancients, and doing what lies in it to give to human morality an essentially selfish character, by disconnecting each man’s feelings of duty from the interests of his fellow-creatures, except so far as a self-interested inducement is offered to him for consulting them. It is essentially a doctrine of passive obedience; it inculcates submission to all authorities found established; who indeed are not to be actively obeyed when they command what religion forbids, but who are not to be resisted, far less rebelled against, for any amount of wrong to ourselves. And while, in the morality of the best Pagan nations, duty to the State holds even a disproportionate place, infringing on the just liberty of the individual; in purely Christian ethics, that grand department of duty is scarcely noticed or acknowledged. It is in the Koran, not the New Testament, that we read the maxim—"A ruler who appoints any man to an office, when there is in his dominions another man better qualified for it, sins against God and against the State.“ What little recognition the idea of obligation to the public obtains in modern morality, is derived from Greek and Roman sources, not from Christian; as, even in the morality of private life, whatever exists of magnanimity, highmindedness, personal dignity, even the sense of honour, is derived from the purely human, not the religious part of our education, and never could have grown out of a standard of ethics in which the only worth, professedly recognised, is that of obedience.

I am as far as any one from pretending that these defects are necessarily inherent in the Christian ethics… Far less would I insinuate this of the doctrines and precepts of Christ himself. I believe that the sayings of Christ are all, that I can see any evidence of their having been intended to be; that they are irreconcilable with nothing which a comprehensive morality requires; that everything which is excellent in ethics may be brought within them… But it is quite consistent with this, to believe that they contain, and were meant to contain, only a part of the truth; that many essential elements of the highest morality are among the things which are not provided for, nor intended to be provided for, in the recorded deliverances of the Founder of Christianity, and which have been entirely thrown aside in the system of ethics erected on the basis of those deliverances by the Christian Church. And this being so, I think it a great error to persist in attempting to find in the Christian doctrine that complete rule for our guidance, which its author intended it to sanction and enforce, but only partially to provide. I believe, too, that this narrow theory is becoming a grave practical evil, detracting greatly from the value of the moral training and instruction, which so many well-meaning persons are now at length exerting themselves to promote. I much fear that by attempting to form the mind and feelings on an exclusively religious type, and discarding those secular standards (as for want of a better name they may be called) which heretofore co-existed with and supplemented the Christian ethics, receiving some of its spirit, and infusing into it some of theirs, there will result, and is even now resulting, a low, abject, servile type of character, which, submit itself as it may to what it deems the Supreme Will, is incapable of rising to or sympathizing in the conception of Supreme Goodness. I believe that other ethics than any one which can be evolved from exclusively Christian sources, must exist side by side with Christian ethics to produce the moral regeneration of mankind; and that the Christian system is no exception to the rule, that in an imperfect state of the human mind, the interests of truth require a diversity of opinions. It is not necessary that in ceasing to ignore the moral truths not contained in Christianity, men should ignore any of those which it does contain…. The exclusive pretension made by a part of the truth to be the whole, must and ought to be protested against; and if a reactionary impulse should make the protestors unjust in their turn, this one-sidedness, like the other, may be lamented, but must be tolerated. If Christians would teach infidels to be just to Christianity, they should themselves be just to infidelity. It can do truth no service to blink the fact, known to all who have the most ordinary acquaintance with literary history, that a large portion of the noblest and most valuable moral teaching has been the work, not only of men who did not know, but of men who knew and rejected, the Christian faith.