1. I Have received the letter which has raeched me under the name of Diodorus, but in what it contains creditable to any one rather than to Diodorus. Some ingenious person seems to have assumed your name, with the intention of getting credit with his hearers. It appears that he was asked by some one if it was lawful to contract marriage with his deceased wife's sister; and, instead of shuddering at such a question, he heard it unmoved, and quite boldly and bravely supported the unseemly desire. Had I his letter by me I would have sent it you, and you would have been able to defend both yourself and the truth. But the person who showed it me took it away again, and carried it about as a kind of trophy of triumph against me who had forbidden it from the beginning, declaring that he had permission in writing. Now I have written to you that I may attack that spurious document with double strength, and leave it no force whereby it may injure its readers.
2. First of all I have to urge, what is of most importance in such matters, our own custom, which has the force of law, because the rules have been handed down to us by holy men. It is as follows: if any one, overcome by impurity, falls into unlawful intercourse with two sisters, this is not to be looked upon as marriage, nor are they to be admitted at all into the Church until they have separated from one another. Wherefore, although it were possible to say nothing further, the custom would be quite enough to safeguard what is right. But, since the writer of the letter has endeavoured to introduce this mischief into our practice by a false argument, I am under the necessity of not omitting the aid of reasoning; although in matters which are perfectly plain every man's instinctive sentiment is stronger than argument.
3. It is written, he says, in Leviticus "Neither shall thou take a wife to her sister, to vex her, to uncover her nakedness. beside the other in her life time."3 From this it is plain, he argues, that it is lawful to take her when the wife is dead. To this my first answer shall be, that whatever the law says, it says to those who are under the law; otherwise we shall be subject to circumcision, the sabbath, abstinence from meats. For we certainly must not, when we find anything which falls in with our pleasures, subject ourselves to the yoke of slavery to the law; and then, if anything in the law seems hard, have recourse to the freedom which is in Christ. We have been asked if it is written that one may be taken to wife after her sister. Let us say what is safe and true, that it is not written. But to deduce by sequence of argument what is passed over in silence is the part of a legislator, not of one who quotes the articles of the law. Indeed, on these terms, any one who likes will be at liberty to take the sister, even in the lifetime of the wife. The same sophism fits in this case also. It is written, he says, "Thou shall not take a wife to vex her:" so that, apart from vexation, there is no prohibition to take her. The man who wants to indulge his desire will maintain that the relationship of sisters is such that they cannot vex one another. Take away the reason given for the prohibition to live with both, and what is there to prevent a man's taking both sisters? This is not written, we shall say. Neither is the former distinctly stated. The deduction from the argument allows liberty in both cases. But a solution of the difficulty might be found by going a little back to what is behind the enactment. It. appears that the legislator does not include every kind of sin, but particularly prohibits those of the Egyptians, from among whom Israel had gone forth, and of the Canaanites among whom they were going. The words are as follows, "After the doings of the land of Egypt, wherein ye dwelt, shall ye not do; and after the doings of the land of Canaan, whither I bring you, shall ye not do: neither shall ye walk in their ordinances."4 It is probable that this kind of sin was not practised at that time among the Gentiles. Under these circumstances the lawgiver was, it may be supposed, under no necessity of guarding against it; the unwritten custom sufficed to condemn the crime. How then is it that while forbidding the greater he was silent about the less? Because the example of the patriarch seemed injurious to many who indulged their flesh so far as to live with sisters in their life time. What ought to be my course? To quote the Scriptures, or to work out what they leave unsaid? In these laws it is not written that a father and son ought not to have the same concubine, but, in the prophet, it is thought deserving of the most extreme condemnation, "A man and his father" it is said "will go in unto the same maid."5 And how many other forms of unclean lust have been found out in the devils' school, while divine scripture is silent about them, not choosing to befoul its dignity with the names of filthy things and condemning their uncleanness in general terms! As the apostle Paul says, "Fornication and all uncleanness ... let it not be once named among you as becometh saints,"6 thus including the unspeakable doings of both males and females under the name of uncleanness. It follows that silence certainly does not give license to voluptuaries.
4. I, however, maintain that this point has not been left in silence, but that the lawgiver has made a distinct prohibition. The words "None of you shall approach to any one that is near of kin to him, to uncover their nakedness,"7 embraces also this form of kinsmanship, for what could be more akin to a man than his own wife, or rather than his own flesh? "For they are no more twain but one flesh."8 So, through the wife, the sister is made akin to the husband. For as he shall not take his wife's mother, nor yet his wife's daughter, because he may not take his own mother nor his own daughter, so he may not take his wife's sister, because he may not take his own sister. And, on the other hand, it will not be lawful for the wife to be joined with the husband's kin, for the rights of relationship hold good on both sides. But, for my part, to every one who is thinking about marriage I testify that, "the fashion of this world passeth away,"9 and the time is short: "it remaineth that both they that have wives be as though they had none."10 If he improperly quotes the charge "Increase and multiply,"11 I laugh at him, for not discerning the signs of the times. Second marriage is a remedy against fornication, not a means of lasciviousness. "If they cannot contain," it is said "let them marry;"12 but if they marry they must not break the law.
5. But they whose souls are blinded by dishonourable lust do not regard even nature, which from old time distinguished the names of the family. For under what relationship will those who contract these unions name their sons? Will they call them brothers or cousins of one another? For, on account of the confusion, both names will apply. O man, do not make the aunt the little one's stepmother; do not arm with implacable jealousy her who ought to cherish them with a mother's love. It is only stepmothers who extend their hatred even beyond death; other enemies make a truce with the dead; stepmothers begin their hatred after death.13 The sum of what I say is this. If any one wants to contract a lawful marriage, the whole world is open to him: if he is only impelled by lust, let him be the more restricted, "that he may know how to possess his vessel in sanctification and honour, not in the lust of concupiscence."14 I should like to say more, but the limits of my letter leave me no further room. I pray that my exhortation may prove stronger than lust, or at least that this pollution may not be found in my own province. Where it has been ventured on there let it abide.