Chapter 15 [XII.]— The One Sin Common to All Men.
But observe more attentively what he says, that “through the offense of one, many are dead.” For why should it be on account of the sin of one, and not rather on account of their own sins, if this passage is to be understood of imitation, and not of propagation? But mark what follows: “And not as it was by one that sinned, so is the gift; for the judgment was by one to condemnation, but the grace is of many offenses unto justification.” Now let them tell us, where there is room in these words for imitation.
“By one,” says he, “to condemnation.” By one what except one sin? This, indeed, he clearly implies in the words which he adds: “But the grace is of many offenses unto justification.” Why, indeed, is the judgment from one offense to condemnation, while the grace is from many offenses to justification? If original sin is a nullity, would it not follow, that not only grace withdraws men from many offenses to justification, but judgment leads them to condemnation from many offenses likewise?
For assuredly grace does not condone many offenses, without judgment in like manner having many offenses to condemn. Else, if men are involved in condemnation because of one offense, on the ground that all the offenses which are condemned were committed in imitation of that one offense; there is the same reason why men should also be regarded as withdrawn from one offense unto justification, inasmuch as all the offenses which are remitted to the justified were committed in imitation of that one offense.
But this most certainly was not the apostle's meaning, when he said: “The judgment, indeed, was from one offense unto condemnation, but the grace was from many offenses unto justification.” We on our side, indeed, can understand the apostle, and see that judgment is predicated of one offense unto condemnation entirely on the ground that, even if there were in men nothing but original sin, it would be sufficient for their condemnation. For however much heavier will be their condemnation who have added their own sins to the original offense (and it will be the more severe in individual cases, in proportion to the sins of individuals); still, even that sin alone which was originally derived unto men not only excludes from the kingdom of God, which infants are unable to enter (as they themselves allow), unless they have received the grace of Christ before they die, but also alienates from salvation and everlasting life, which cannot be anything else than the kingdom of God, to which fellowship with Christ alone introduces us.
Source: Merits and Remission of Sin, and Infant Baptism (New Advent)