Nicene and Post-Nicene Fathers, Vol. V:
On Marriage and Concupiscence.: Chapter 27

Chapter 27 [XXIV.]—Through Lust Original Sin is Transmitted; Venial Sins in Married Persons; Concupiscence of the Flesh, the Daughter and Mother of Sin.

Wherefore the devil holds infants guilty who are born, not of the good by which marriage is good, but of the evil of concupiscence, which, indeed, marriage uses aright, but at which even marriage has occasion to feel shame. Marriage is itself “honourable in all” 2151 the goods which properly appertain to it; but even when it has p. 275 its “bed undefiled” (not only by fornication and adultery, which are damnable disgraces, but also by any of those excesses of cohabitation such as do not arise from any prevailing desire of children, but from an overbearing lust of pleasure, which are venial sins in man and wife), yet, whenever it comes to the actual process of generation, the very embrace which is lawful and honourable cannot be effected without the ardour of lust, so as to be able to accomplish that which appertains to the use of reason and not of lust. Now, this ardour, whether following or preceding the will, does somehow, by a power of its own, move the members which cannot be moved simply by the will, and in this manner it shows itself not to be the servant of a will which commands it, but rather to be the punishment of a will which disobeys it. It shows, moreover, that it must be excited, not by a free choice, but by a certain seductive stimulus, and that on this very account it produces shame. This is the carnal concupiscence, which, while it is no longer accounted sin in the regenerate, yet in no case happens to nature except from sin. It is the daughter of sin, as it were; and whenever it yields assent to the commission of shameful deeds, it becomes also the mother of many sins. Now from this concupiscence whatever comes into being by natural birth is bound by original sin, unless, indeed, it be born again in Him whom the Virgin conceived without this concupiscence. Wherefore, when He vouchsafed to be born in the flesh, He alone was born without sin.

Footnotes

Heb. xiii. 4.