Augustine of Hippo, On the Merits and Forgiveness of Sins, and on the Baptism of Infants III.9: The Ambiguity of 'Adam is the Figure of Him to Come.'

To me, however, no doubt presents itself about the whole of this passage, in which the apostle speaks of the condemnation of many through the sin of one, and the justification of many through the righteousness of One, except as to the words, “Adam is the figure of Him that was to come.” For this phrase in reality not only suits the sense which understands that Adam’s posterity were to be born of the same form as himself along with sin, but the words are also capable of being drawn out into several distinct meanings. For we have ourselves perhaps actually contended for various senses from the words in question at different times, and very likely we shall propound yet another view, which, however, will not be incompatible with the sense here mentioned; and even Pelagius has not always expounded the passage in one way. All the rest, however, of the passage in which these doubtful words occur, if its statements are carefully examined and treated, as I have tried my best to do in the first book of this treatise, will not (in spite of the obscurity of style necessarily engendered by the subject itself) fail to show the incompatibility of any other meaning than that which has secured the adhesion of the universal Church from the earliest times—that believing infants have obtained through the baptism of Christ the remission of original sin.