Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 16

Chapter 16.—Of Marriage Between Blood-Relations, in Regard to Which the Present Law Could Not Bind the Men of the Earliest Ages.

As, therefore, the human race, subsequently to the first marriage of the man who was made of dust, and his wife who was made out of his side, required the union of males and females in order that it might multiply, and as there were no human beings except those who had been born of these two, men took their sisters for wives,—an act which was as certainly dictated by necessity in these ancient days as afterwards it was condemned by the prohibitions of religion.  For it is very reasonable and just that men, among whom concord is honorable and useful, should be bound together by various relationships; and one man should not himself sustain many relationships, but that the various relationships should be distributed among several, and should thus serve to bind together the greatest number in the same social interests.  “Father” and “father-in-law” are the names of two relationships.  When, therefore, a man has one person for his father, another for his father-in-law, friendship extends itself to a larger number.  But Adam in his single person was obliged to hold both relations to his sons and daughters, for brothers and sisters were united in marriage.  So too Eve his wife was both mother and mother-in-law to her children of both sexes; while, had there been two women, one the mother, the other the mother-in-law, the family affection would have had a wider field.  Then the sister herself by becoming a wife sustained in her single person two relationships, which, had they been distributed among individuals, one being sister, and another being wife, the family tie would have embraced a greater number of persons.  But there was then no material for effecting this, since there were no human beings but the brothers and sisters born of those two first parents.  Therefore, when an abundant population made it possible, men ought to choose for wives women who were not already their sisters; for not only would there then be no necessity for marrying sisters, but, were it done, it would be most abominable.  For if the grandchildren of the first pair, being now able to choose their cousins for wives, married their sisters, then it would no longer be only two but three relationships that were held by one man, while each of these relationships ought to have been held by a separate individual, so as to bind together by family affection a larger number.  For one man would in that case be both father, and father-in-law, and uncle 818 to his own children (brother and sister now man and wife); and his wife would be mother, aunt, and mother-in-law to them; and they themselves would be not only brother and sister, and man and wife, but cousins also, being the children of brother and sister.  Now, all these relationships, which combined three men into one, would have embraced nine persons had each relationship been held by one individual, so that a man had one person for his sister, another his wife, another his cousin, another his father, another his uncle, another his father-in-law, another his mother, another his aunt, another his mother-in-law; and thus the social bond would not have been tightened to bind a few, but loosened to embrace a larger number of relations.

And we see that, since the human race has increased and multiplied, this is so strictly observed even among the profane worshippers of many and false gods, that though their laws perversely allow a brother to marry his sister, 819 yet custom, with a finer morality, prefers to forego this license; and though it was quite allowable in the earliest ages of the human race to marry one’s sister, it is now abhorred as a thing which no circumstances could justify.  For custom has very great power either to attract or to shock human feeling.  And in this matter, while it restrains concupiscence p. 298 within due bounds, the man who neglects and disobeys it is justly branded as abominable.  For if it is iniquitous to plough beyond our own boundaries through the greed of gain, is it not much more iniquitous to transgress the recognized boundaries of morals through sexual lust?  And with regard to marriage in the next degree of consanguinity, marriage between cousins, we have observed that in our own time the customary morality has prevented this from being frequent, though the law allows it.  It was not prohibited by divine law, nor as yet had human law prohibited it; nevertheless, though legitimate, people shrank from it, because it lay so close to what was illegitimate, and in marrying a cousin seemed almost to marry a sister,—for cousins are so closely related that they are called brothers and sisters, 820 and are almost really so.  But the ancient fathers, fearing that near relationship might gradually in the course of generations diverge, and become distant relationship, or cease to be relationship at all, religiously endeavored to limit it by the bond of marriage before it became distant, and thus, as it were, to call it back when it was escaping them.  And on this account, even when the world was full of people, though they did not choose wives from among their sisters or half-sisters, yet they preferred them to be of the same stock as themselves.  But who doubts that the modern prohibition of the marriage even of cousins is the more seemly regulation—not merely on account of the reason we have been urging, the multiplying of relationships, so that one person might not absorb two, which might be distributed to two persons, and so increase the number of people bound together as a family, but also because there is in human nature I know not what natural and praiseworthy shamefacedness which restrains us from desiring that connection which, though for propagation, is yet lustful and which even conjugal modesty blushes over, with any one to whom consanguinity bids us render respect?

The sexual intercourse of man and woman, then, is in the case of mortals a kind of seed-bed of the city; but while the earthly city needs for its population only generation, the heavenly needs also regeneration to rid it of the taint of generation.  Whether before the deluge there was any bodily or visible sign of regeneration, such as was afterwards enjoined upon Abraham when he was circumcised, or what kind of sign it was, the sacred history does not inform us.  But it does inform us that even these earliest of mankind sacrificed to God, as appeared also in the case of the two first brothers; Noah, too, is said to have offered sacrifices to God when he had come forth from the ark after the deluge.  And concerning this subject we have already said in the foregoing books that the devils arrogate to themselves divinity, and require sacrifice that they may be esteemed gods, and delight in these honors on no other account than this, because they know that true sacrifice is due to the true God.

Footnotes

His own children being the children of his sister, and therefore his nephews.

This was allowed by the Egyptians and Athenians, never by the Romans.

Both in Hebrew, Greek, and Latin, though not uniformly, nor in Latin commonly.