And if they are not to be blamed for so doing, let us see whether Christians do not exhort multitudes to the practice of virtue in a greater and better degree than they. For the philosophers who converse in public do not pick and choose their hearers, but he who likes stands and listens. The Christians, however, having previously, so far as possible, tested the souls of those who wish to become their hearers, and having previously instructed them in private, when they appear (before entering the community) to have sufficiently evinced their desire towards a virtuous life, introduce them then, and not before, privately forming one class of those who are beginners, and are receiving admission, but who have not yet obtained the mark of complete purification; and another of those who have manifested to the best of their ability their intention to desire no other things than are approved by Christians; and among these there are certain persons appointed to make inquiries regarding the lives and behaviour of those who join them, in order that they may prevent those who commit acts of infamy from coming into their public assembly, while those of a different character they receive with their whole heart, in order that they may daily make them better. And this is their method of procedure, both with those who are sinners, and especially with those who lead dissolute lives, whom they exclude from their community, although, according to Celsus, they resemble those who in the market-places perform the most shameful tricks. Now the venerable school of the Pythagoreans used to erect a cenotaph to those who had apostatized from their system of philosophy, treating them as dead; but the Christians lament as dead those who have been vanquished by licentiousness or any other sin, because they are lost and dead to God, and as being risen from the dead (if they manifest a becoming change) they receive them afterwards, at some future time, after a greater interval than in the case of those who were admitted at first, but not placing in any office or post of rank in the Church of God those who, after professing the Gospel, lapsed and fell.
Source: Against Celsus (New Advent)