10 “For, behold, in iniquities I was conceived”. 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. David has taken upon him the person of mankind, and has heeded the bonds of all men, has considered the offspring of death, has adverted to the origin of iniquity, and he says, “For, behold, in iniquities I was conceived.” Was David born of adultery; being born of Jesse, a righteous man, and his own wife? What is it that he says himself to have been in iniquity conceived, except that iniquity is drawn from Adam? Even the very bond of death, with iniquity itself is engrained? No man is born without bringing punishment, bringing desert of punishment. A Prophet says also in another place, “No one is clean in Your sight, not even an infant, whose life is of one day upon earth.” For we know both by the Baptism of Christ that sins are loosed, and that the Baptism of Christ avails the remission of sins. If infants are every way innocent, why do mothers run with them when sick to the Church? What by that Baptism, what by that remission is put away? An innocent one I see that rather weeps than is angry. What does Baptism wash off? What does that Grace loose? There is loosed the offspring of sin. For if that infant could speak to you, it would say, and if it had the understanding which David had, it would answer you, Why do you heed me, an infant? Thou dost not indeed see my actions: but I in iniquity have been conceived, “And in sins has my mother nourished me in the womb.”
Apart from this bond of mortal concupiscence was Christ born without a male, of a virgin conceiving by the Holy Ghost. He cannot be said to have been conceived in iniquity, it cannot be said, In sins His mother nourished Him in the womb, to whom was said, “The Holy Ghost shall come upon you, and the Virtue of the Highest shall overshadow you.” It is not therefore because it is sin to have to do with wives that men are conceived in iniquity, and in sins nourished in the womb by their mother; but because that which is made is surely made of flesh deserving punishment. For the punishment of the flesh is death, and surely there is in it liability to death itself. Whence the Apostle spoke not of the body as if to die, but as if dead: “The body indeed is dead,” he says, “because of sin, but the Spirit is life because of righteousness.” How then without bond of sin is born that which is conceived and sown of a body dead because of sin? This chaste operation in a married person has not sin, but the origin of sin draws with it condign punishment. For there is no husband that, because he is an husband, is not subject to death, or that is subject to death for any other reason but because of sin. For even the Lord was subject to death, but not on account of sin: He took upon Him our punishment, and so looses our guilt. With reason then, “In Adam all die, but in Christ shall all be made alive.” For, “Through one man,” says the Apostle, “sin has entered into this world, and through sin death, and so has passed unto all men, in that all have sinned.” Definite is the sentence: “In Adam,” he says, “all have sinned.” Alone then could such an infant be innocent, as has not been born of the work of Adam.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)