Now, that miraculous appearances have sometimes been witnessed by human beings, is related by the Greeks; and not only by those of them who might be suspected of composing fabulous narratives, but also by those who have given every evidence of being genuine philosophers, and of having related with perfect truth what had happened to them. Accounts of this kind we have read in the writings of Chrysippus of Soli, and also some things of the same kind relating to Pythagoras; as well as in some of the more recent writers who lived a very short time ago, as in the treatise of Plutarch of Chæronea “on the Soul,” and in the second book of the work of Numenius the Pythagorean on the “Incorruptibility of the Soul.”
Now, when such accounts are related by the Greeks, and especially by the philosophers among them, they are not to be received with mockery and ridicule, nor to be regarded as fictions and fables; but when those who are devoted to the God of all things, and who endure all kinds of injury, even to death itself, rather than allow a falsehood to escape their lips regarding God, announce the appearances of angels which they have themselves witnessed, they are to be deemed unworthy of belief, and their words are not to be regarded as true!
Now it is opposed to sound reason to judge in this way whether individuals are speaking truth or falsehood. For those who act honestly, only after a long and careful examination into the details of a subject, slowly and cautiously express their opinion of the veracity or falsehood of this or that person with regard to the marvels which they may relate; since it is the case that neither do all men show themselves worthy of belief, nor do all make it distinctly evident that they are relating to men only fictions and fables.
Moreover, regarding the resurrection of Jesus from the dead, we have this remark to make, that it is not at all wonderful if, on such an occasion, either one or two angels should have appeared to announce that Jesus had risen from the dead, and to provide for the safety of those who believed in such an event to the advantage of their souls. Nor does it appear to me at all unreasonable, that those who believe in the resurrection of Jesus, and who manifest, as a fruit of their faith not to be lightly esteemed, their possession of a virtuous life, and their withdrawal from the flood of evils, should not be unattended by angels who lend their help in accomplishing their conversion to God.
Source: Against Celsus (New Advent)