The Enchiridion

And it is said, with much appearance of probability, that infants are involved in the guilt of the sins not only of the first pair, but of their own immediate parents. For that divine judgment, “I shall visit the iniquities of the fathers upon the children,” certainly applies to them before they come under the new covenant by regeneration. And it was this new covenant that was prophesied of, when it was said by Ezekiel, that the sons should not bear the iniquity of the fathers, and that it should no longer be a proverb in Israel, “The fathers have eaten sour grapes, and the children's teeth are set on edge.” Here lies the necessity that each man should be born again, that he might be freed from the sin in which he was born.

For the sins committed afterwards can be cured by penitence, as we see is the case after baptism. And therefore the new birth would not have been appointed only that the first birth was sinful, so sinful that even one who was legitimately born in wedlock says: “I was shapen in iniquities, and in sins did my mother conceive me.” He did not say in iniquity, or in sin, though he might have said so correctly; but he preferred to say “iniquities” and “sins,” because in that one sin which passed upon all men, and which was so great that human nature was by it made subject to inevitable death, many sins, as I showed above, may be discriminated; and further, because there are other sins of the immediate parents, which though they have not the same effect in producing a change of nature, yet subject the children to guilt unless the divine grace and mercy interpose to rescue them.