Epictetus uses the Christians as an example of freedom from fear (Discourses 4.7):
What makes the tyrant formidable? "The guards," you say, "and their swords, and the men of the bedchamber and those who exclude them who would enter." Why, then, if you bring a boy to the tyrant when he is with his guards, is he not afraid; or is it because the child does not understand these things? If, then, any man does understand what guards are and that they have swords, and comes to the tyrant for this very purpose because he wishes to die on account of some circumstance and seeks to die easily by the hand of another, is he afraid of the guards? "No, for he wishes for the thing which makes the guards formidable." If, then, neither any man wishing to die nor to live by all means, but only as it may be permitted, approaches the tyrant, what hinders him from approaching the tyrant without fear? "Nothing." If, then, a man has the same opinion about his property as the man whom I have instanced has about his body; and also about his children and his wife, and in a word is so affected by some madness or despair that he cares not whether he possesses them or not, but like children who are playing, with shells care about the play, but do not trouble themselves about the shells, so he too has set no value on the materials, but values the pleasure that he has with them and the occupation, what tyrant is then formidable to him or what guards or what swords?
Then through madness is it possible for a man to be so disposed toward these things, and the Galilaens through habit, and is it possible that no man can learn from reason and from demonstration that God has made all the things in the universe and the universe itself completely free from hindrance and perfect, and the parts of it for the use of the whole?
Thus we see the Galileans, who are almost certainly to be identified with the Christians, classified as a group that overcomes freedom from fear "through habit". Epictetus regards freedom from fear as a good, of course, but there is an implicit criticism here, that the Galileans have obtained freedom from fear in a problematic way: if even madmen obtain freedom from fear through insanity, and even Christians obtain freedom through disposition, how can it be a that a rational man cannot obtain freedom through reason?
This criticism, implicit in Epictetus, is made more explicit in Marcus Aurelius (Meditations 11.3):
What a soul that is which is ready, if at any moment it must be separated from the body, and ready either to be extinguished or dispersed or continue to exist; but so that this readiness comes from a man's own judgement, not from mere obstinacy, as with the Christians, but considerately and with dignity and in a way to persuade another, without tragic show.
There seems to be a school of thought that holds that 'as with the Christians' is a later interpolation; regardless, somebody of Stoic tastes thought that the Christians were a good contrast case here. The word translated as obstinacy here, parataxis, seems to have been used to the marshalling of troops or drumming up of support in politics: it's not just that the Christians are unafraid of death, it's that they are trained to want to die, and with tragic show. The Stoics held that suicide was good if done out of a deliberate and honorable choice; but Christian martyrdom was to them a bizarre and unacceptable thing, because of the brazenness of it. It was not death with gravitas but an obstinate headlong plunge that they encouraged in each other.
The obstinacy of Christians had actually become almost proverbial; the Stoics are not the only ones to mention it. We have a fragment from Galen in which he mentions it; he notes that Christian contempt for death comes from their following of parables (apparently stories about the afterlife) rather than reason, and this may, indeed, be what was in view in the passage in the Meditations, as well.
