St. Augustine of Hippo (354-430) in Book II of his De nuptiis et concupiscentia (On Marriage and Concupiscence) explains how original sin is transmitted from one generation to the next, beginning with Adam and Eve.
Some would trace the cause of original sin to the infant, or to the institution of marriage, or to the parents. But St. Augustine rejects these notions. As for the infant, he is not mature enough to sin. As for the marriage, it is an institution established by God Himself, not for sin, but for a good purpose, for He said, “Be fruitful, and multiply, and replenish the earth” (Gn 1:28). As for the parents who are lawfully joined in the matrimonial bond, they carry out God’s directive for the procreation of children, for the propagation of the human race.
Citing the authority of St. Paul, Augustine writes: “Now, there is an answer for him to all these questions given by the Apostle, who censures neither the infant’s will, which is not yet matured in him for sinning, nor marriage, which, as such, has not only its institution, but its blessing also, from God; nor parents, so far as they are parents, who are united together properly and lawfully for the procreation of children; but he says, ‘By one man sin entered into the world, and death by sin; and so death passed upon all men for in him all have sinned’ (Rom 5:12).” That is to say, St. Paul identifies Adam as the cause of original sin, for through his voluntary act of disobedience, sin first entered into our world.
Some incorrectly interpret this verse (Rom 5:12) to mean “that Adam was the first to sin, and that any one who wished afterwards to commit sin found an example for sinning in him; so that sin . . . did not pass from this one upon all men by birth, but by the imitation of this one.” But, St. Augustine explains: “It is certain that if the apostle meant this imitation to be here understood, he would have said that sin had entered into the world and passed upon all men, not by one man, but rather by the devil. For of the devil it is written: ‘They that are on his side do imitate him’ (Wis 2:24). He used the phrase ‘by one man,’ from whom the generation of men, of course, had its beginning, in order to show us that original sin had passed upon all men by generation.” (II, 45)
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Vol. V, ed. Philip Schaff (Edinburgh: T&T Clark, 1886).