That Original Sin is transmitted from our First Parent to his Posterity

THIS expressly appears from the words of the Apostle: As by one man sin came into the world, and by sin death, so death passed on to all men, seeing that all have sinned (Rom. V, 12)*. It cannot be said that by one man sin entered into the world by way of imitation, because in that interpretation sin would have reached only to those who imitate the first man in sinning; and since by sin death came into the world, death would reach only those who sin in the likeness of the first man that sinned. But to exclude this interpretation the Apostle adds: Death reigned from Adam to Moses even over those who did not sin in the likeness of the transgression of Adam. The Apostle's meaning therefore was not that by one man sin entered into the world in the way of imitation, but in the way of origin.

Moreover, the common custom of the Church is to administer baptism to new-born children. But there would be no purpose in such administration, unless there were sin in them. If it is said that the purpose of infant baptism is not the cleansing of sin, but the arriving at the kingdom of God, the saying is nonsensical. They who say so, appeal to our Lord's words: Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God (John iii, 5).* The fact is, no one is excluded from the kingdom of God except for some fault. For the end of every rational creature is to arrive at happiness; which happiness can be only in the kingdom of God; which kingdom again is nothing else than the organised society of those who enjoy the vision of God, in which true happiness consists (B. III, Chap. LXIII).* But nothing fails to gain its end except through some fault or flaw. If then unbaptised children cannot arrive at the kingdom of God, we must say that there is some fault, flaw, or sin in them.

4.55 : Points of Reply to Difficulties touching the Economy of the Incarnation
4.51, 52 : Arguments against Original Sin, with Replies