Augustine of Hippo

"For, behold, in iniquities I was conceived" [Psalm 51:5]. As though he were saying, They are conquered that have done what thou, David, hast done: for this is not a little evil and little sin, to wit, adultery and man-slaying. What of them that from the day that they were born of their mother's womb, have done no such thing? Even to them do you ascribe some sins, in order that He may conquer all men when He begins to be judged. …

Exposition on Psalm 51 ↗

Jerome

But we, according to the epistle of James, "all stumble in many things," and "no one is pure from sin, no not if his life is but a day long." For who will boast "that he has a clean heart? or who will be sure that he is pure from sin?" And we are held guilty after the likeness of Adam's transgression. Hence David says, "Behold, I was shaped in iniquity, and in sin did my mother conceive me." And the blessed Job, "Even if I were righteous, my mouth will speak wickedness; even if I were perfect, I will be found guilty. …

Against Jovinianus 2.2 ↗

Thomas Aquinas

"Behold indeed." Here he sets forth the root of the guilt. The root of all actual guilt is original sin, which is contracted from parents infected with that sin; this infection was in the father of David and in his mother. As to the father, he says, "in iniquities I was conceived" -- not actual iniquities, because he was not born of adultery, but of marriage and from holy Jesse, as is said in the last chapter of Ruth -- but in original sin; for in this sin all are born. Rom. …

Exposition on the Psalms of David ↗