31 But what is the precise value of the sanctification of the sacrament (which that thief did not receive, not from any want of will on his part, but because it was unavoidably omitted) and what is the effect on a man of its material application, it is not easy to say. Still, had it not been of the greatest value, the Lord would not have received the baptism of a servant. But since we must look at it in itself, without entering upon the question of the salvation of the recipient, which it is intended to work, it shows clearly enough that both in the bad, and in those who renounce the world in word and not in deed, it is itself complete, though they cannot receive salvation unless they amend their lives.
But as in the thief, to whom the material administration of the sacrament was necessarily wanting, the salvation was complete, because it was spiritually present through his piety, so, when the sacrament itself is present, salvation is complete, if what the thief possessed be unavoidably wanting. And this is the firm tradition of the universal Church, in respect of the baptism of infants, who certainly are as yet unable "with the heart to believe unto righteousness, and with the mouth to make confession unto salvation," as the thief could do; nay, who even, by crying and moaning when the mystery is performed upon them, raise their voices in opposition to the mysterious words, and yet no Christian will say that they are baptized to no purpose.
Source: On Baptism, Against the Donatists (New Advent)