That philosophical discourses, however, distinguished by orderly arrangement and elegant expression, should produce such results in the case of those individuals just enumerated, and upon others who have led wicked lives, is not at all to be wondered at. But when we consider that those discourses, which Celsus terms “vulgar,” are filled with power, as if they were spells, and see that they at once convert multitudes from a life of licentiousness to one of extreme regularity, and from a life of wickedness to a better, and from a state of cowardice or unmanliness to one of such high-toned courage as to lead men to despise even death through the piety which shows itself within them, why should we not justly admire the power which they contain?
For the words of those who at the first assumed the office of (Christian) ambassadors, and who gave their labours to rear up the Churches of God—nay, their preaching also—were accompanied with a persuasive power, though not like that found among those who profess the philosophy of Plato, or of any other merely human philosopher, which possesses no other qualities than those of human nature. But the demonstration which followed the words of the apostles of Jesus was given from God, and was accredited by the Spirit and by power.
And therefore their word ran swiftly and speedily, or rather the word of God through their instrumentality, transformed numbers of persons who had been sinners both by nature and habit, whom no one could have reformed by punishment, but who were changed by the word, which moulded and transformed them according to its pleasure.
Source: Against Celsus (New Advent)