But since he is manifestly guilty of falsehood in the statements which follow, let us examine his assertion when he says, “If all men wished to become Christians, the latter would not desire such a result.” Now that the above statement is false is clear from this, that Christians do not neglect, as far as in them lies, to take measures to disseminate their doctrine throughout the whole world. Some of them, accordingly, have made it their business to itinerate not only through cities, but even villages and country houses, that they might make converts to God.
And no one would maintain that they did this for the sake of gain, when sometimes they would not accept even necessary sustenance; or if at any time they were pressed by a necessity of this sort, were contented with the mere supply of their wants, although many were willing to share (their abundance) with them, and to bestow help upon them far above their need. At the present day, indeed, when, owing to the multitude of Christian believers, not only rich men, but persons of rank, and delicate and high-born ladies, receive the teachers of Christianity, some perhaps will dare to say that it is for the sake of a little glory that certain individuals assume the office of Christian instructors.
It is impossible, however, rationally to entertain such a suspicion with respect to Christianity in its beginnings, when the danger incurred, especially by its teachers, was great; while at the present day the discredit attaching to it among the rest of mankind is greater than any supposed honour enjoyed among those who hold the same belief, especially when such honour is not shared by all. It is false, then, from the very nature of the case, to say that “if all men wished to become Christians, the latter would not desire such a result.”
Source: Against Celsus (New Advent)