This timidity, however, could not prevent the rise of a Pelagius with his profane fiction—that Adam sinned only to his own hurt, but did no hurt to his posterity. Satan, by thus craftily hiding the disease, tried to render it incurable. But when it was clearly proved from Scripture that the sin of the first man passed to all his posterity, recourse was had to the cavil, that it passed by imitation, and not by propagation. The orthodoxy, therefore, and more especially Augustine, laboured to show, that we are not corrupted by acquired wickedness, but bring an innate corruption from the very womb. It was the greatest impudence to deny this. But no man will wonder at the presumption of the Pelagians and Celestians, who has learned from the writings of that holy man how extreme the effrontery of these heretics was.
Surely there is no ambiguity in David’s confession, “I was shapen in iniquity; and in sin did my mother conceive me,” (Ps. 51:5). His object in the passage is not to throw blame on his parents; but the better to commend the goodness of God towards him, he properly reiterates the confession of impurity from his very birth. As it is clear, that there was no peculiarity in David’s case, it follows that it is only an instance of the common lot of the whole human race. All of us, therefore, descending from an impure seed, come into the world tainted with the contagion of sin. Nay, before we behold the light of the sun we are in God’s sight defiled and polluted. “Who can bring a clean thing out of an unclean? Not one,” says the Book of Job (Job 14:4).
John Calvin, Institutes of the Christian Religion (Bellingham, WA: Logos Bible Software, 1997).