From the Canons of St. Basil, in the Letters to Amphilochius, A.D. 374-75

Consider that these canons are of themselves penitential or penal. They were indeed handed down by private authority; yet they were in the highest honor among the Greeks. They exhibit, generally, the canon law then in force in the Greek Church concerning those matters about which Basil had been questioned. Wherefore we could not abstain from reviewing these canons.

Canon III. A deacon who has fornicated after the diaconate shall indeed be cast out from the diaconate; but, thrust out into the place of laymen, he will not be kept from communion, since it is an ancient canon that those who have fallen from a grade are punished by this mode of punishment alone.

If you compare this canon with the following Canon XLIV, where no further carnal use is permitted to deaconesses, as consecrated, and therefore not even conjugal use, it will be evident that this was much more forbidden to deacons. Wherefore by the name of fornication Basil understands any carnal use whatever, even conjugal use, which is illicit for deacons.

Canon IV. They defined the same canon for trigamists and polygamists as for digamists, proportion being preserved; for some appoint one year for digamists, but others two, while they often segregate trigamists for three and four years. But they call trigamy no longer marriage, but polygamy, or rather fornication under constraint ... But by custom we have received a segregation of five years in the case of trigamists, not from the canons but from the consequence of those things which had previously been received. But it is necessary not to keep them wholly from the Church, but to admit them among the hearers for two or three years, and afterwards to receive them among the standers, yet not to admit them to the communion of the Good; and thus, after they have shown some fruit of penance, to restore them to the place of communion.

Canon VI. Let the fornication of Τῶν κανονικῶν not be reckoned as marriage, but let their union be wholly torn apart.

The Maurist editors, whom see, contend that the speech is only about women and that canonicae, or sacred virgins, are understood. Cf. Canon XIX of the same Basil.

Canon IX. According to the consequence of the sentence, what was pronounced by the Lord, that it is not lawful to depart from marriage except because of fornication, applies equally to men and women.

Canon XII. The canon has wholly excluded digamists from ministry.

Canon XVIII. Concerning fallen virgins, who have professed to the Lord a chaste life, and afterwards, overcome by the lusts of the flesh, make their covenants void, our fathers, since they accommodated themselves simply and gently to the weakness of those who fall, judged that these could be admitted after a year, judging about them according to the likeness of digamists. But indeed it seems to me, since by the gift of God the Church, progressing, becomes stronger, and now the order of virgins is multiplied, that the matter itself must be carefully considered, as it is manifest to one considering it, and the sentence of Scripture, which can be found from consequence. For widowhood is inferior to virginity; therefore also the offense of widows is far less than that of virgins. Let us therefore see what was written to Timothy by Paul, 1 Tim. V. 11-12. And so if a widow is subjected to a most grave judgment as one who has made void her faith in Christ, what must we think about a virgin, who is the spouse of Christ and a sacred vessel dedicated to the Lord? It is indeed a great sin even for a maidservant, giving herself to clandestine marriages, to fill the house with debaucheries and by a worse life to do injury to her owner; but it is far more grave for a spouse to become an adulteress and, bringing disgrace to her union with the spouse, to give herself to immodest pleasures. Therefore the widow indeed is condemned as a corrupted maidservant, but the virgin is subjected to the judgment of an adulteress. Therefore, just as we call him who is with another's wife an adulterer, not admitting him to communion before he has ceased from the sin, so certainly we shall also establish concerning him who has a virgin. But we must establish this in advance: that she is called a virgin who has offered herself of her own accord to the Lord, and has renounced nuptials, and has embraced the institute of sanctity. But we admit professions from that time at which age has had the completion of reason ... she who, being more than sixteen or seventeen years old, mistress of her own reasonings, after being examined for a long time and proved, has thereafter persevered and has constantly asked to be admitted, then at last is to be numbered among virgins, and her profession must be held ratified and the violation of it inexorably punished.

You have a clear definition of a professed virgin. Basil proposes his opinion concerning the fornication of these virgins, indicating sufficiently that it had not yet been received into use in the Greek Church. But that by the violation of the covenant, which he calls adultery, Basil also comprehends attempted marriage seems to me at least very probable from the universal formulas which he uses.

Canon XIX. But we do not know a profession of men, except if any have enrolled themselves in the order of monks, who indeed seem tacitly to have undertaken a celibate life. Nevertheless in these also I think that this ought to be placed first, that they themselves be questioned and their clear and manifest profession be received, so that when they have betaken themselves to a lustful and voluptuous life, they may be subjected to the punishment of those who fornicate.

Basil wishes, in this canon and the preceding canon, that profession be entirely free in those who are held by it. Cf. below, Canon XLIX of the fourth Council of Toledo, A.D. 633.

It can be asked what Basil means when he says that the Greeks do not know a profession of men, since he has known male monks and, in Canon LX, wishes that they be held by the same law as professed virgins when they fall. The difference, namely, was in this: that virgins, embracing the state of nuns, made profession before the Bishop and were consecrated by him with a certain sacred rite. This was not used for men, who, by enrolling themselves in the order of monks, seemed by that very fact tacitly to have undertaken a celibate life, as is said here.

Canon XXI. If a man, dwelling with his wife, afterwards not content with marriage, has fallen into fornication, we judge him to be a fornicator and we prolong him further in the imposed penalties; but nevertheless we do not have a canon which subjects him to the crime of adultery (that is, punishing him with the penalty of an adulterer), if the sin has been committed with one free from marriage, because he who has fornicated is not excluded from dwelling with his wife. Wherefore the wife indeed will receive her husband returning from fornication; but the husband will cast the polluted woman out of his house. And indeed the reason of these things is not easy, but custom has thus prevailed.

Cf. from the letter of Innocent I to Exuperius, decree IV.

Canon XXII. Those who have women by abduction, if indeed they have carried off women already betrothed to others, are not to be admitted before they have been taken away from them and restored to the power of those to whom they had been betrothed from the beginning, whether they wish to receive them or desist. But if someone has taken one who is free, it is necessary indeed to take her away and restore her to her own people and permit it to their will ... and if they wish to hand her over to him (that is, to the abductor), it is necessary to establish the marriage; but if they hold her back, by no means to inflict force.

Cf. Canon XI of the Council of Ancyra.

Canon XXIII. But concerning those who marry two sisters, or concerning those women who marry two brothers, a letter has been issued by us, a copy of which we have sent to your piety. But he who has taken his brother's wife is not to be admitted before he has withdrawn from her.

The letter to which he appeals is CLX (another numbering: 197) to Diodorus. There, after the question has been proposed whether it is lawful for a man to marry the sister of his dead wife, which Basil had already prohibited at the beginning of his episcopate, he defends his negative opinion. The first argument of this defense is: "First, what is greatest in things of this sort, we can put forward our custom as having the force of law, because the rules have been handed down to us by holy men. But that custom is of this sort: that if anyone, at some time overcome by the vice of impurity, has fallen into the illicit union of two sisters, neither is that judged a marriage, nor are they at all admitted into the assembly of the Church before they are separated from one another." Cf. Canon II of the Council of Neocaesarea.

Canon XXV. He who keeps as wife one defiled by himself will indeed undergo the penalty of defilement, but it will be lawful for him to have her as wife.

Canon XXXI. She whose husband has departed and does not appear, dwelling together with another before she has been made more certain of his death, commits adultery.

Canon XXXV. In the case of a husband deserted by his wife, the cause of desertion must be considered; and if it has been established that she departed without reason, he indeed is worthy of pardon, but she of a fine. Pardon, however, will be given to him (the husband), so that he may communicate with the Church.

You see that another marriage is by no means permitted while the wife who departed without reason is living.

Canon XXXVI. Wives of soldiers who have married when their husbands did not appear are subject to the same reckoning as those also who, because of the travel of their husbands, did not await their return (Canon XXXI); but nevertheless the matter admits here no little pardon (with respect to imposing penance), because there is a greater suspicion of death.

Canon XXXVIII. Girls who have followed contrary to the judgment of the father commit fornication (cf. the following Canon XL). But when the parents have been reconciled, the matter seems to receive a remedy. Yet they are not immediately restored to communion, but will be punished for three years.

Canon XL. She who, contrary to the judgment of her master, hands herself over to a man has fornicated; but she who afterwards has used free marriage has married. For the covenants of those who are in the power of another have nothing firm.

Since marriage is from the right of nature, the Church did not receive this prescription, nor the preceding and following, with respect to the value of marriage, nor for any case whatever with respect to honesty.

Canon XLII. Marriages without those who have power are fornications. Therefore neither while the father is living, nor while the master is living, are those who come together beyond reproach. In the same way, if those in whose power the arbitration of this matter lies consent to the cohabitation, then the cohabitation receives the firmness of marriage.

Canon XLIV. We do not permit the body of a deaconess, as consecrated, to be any further in carnal use.

Canon XLVIII. She who has been left by her husband, in my opinion indeed, ought to remain. For if the Lord said, "If anyone leaves his wife, except for the cause of fornication, he makes her commit adultery," from the fact that He calls her an adulteress, He has shut off for her union with another. For how could the man indeed be guilty as the cause of adultery, while the woman be without fault, she who has been called an adulteress by the Lord because of union with another?

Cf. Thesis XXI, V.

Canon L. There is no law of trigamy (approving it). Wherefore by law a third marriage is not contracted. Yet we indeed hold such things as pollutions of the Church, but we do not subject them to public condemnations, as being more to be chosen than loose fornication.

Canon LX. She who, having professed virginity, has fallen from her purpose, will complete the time of the sin of adultery in the prescription of continence. The same also applies to those who have professed the monastic life and fall.

Canon LXXV. He who has been polluted with his sister from his father or mother (that is, the daughter of a stepmother or stepfather) is not to be permitted to approach the house of prayer until he has desisted from the wicked and nefarious action. But after he has come to the sense and recognition of the horrid sin, let him weep for three years, etc.

Between you and the daughter of your stepmother there is not affinity of the first kind, but of the other; nevertheless Basil wished this marriage to be null.

Canon LXXVI. The same reasoning also applies to those who take their daughters-in-law.

Nuptials with a daughter-in-law are judged illicit by Basil by reason of affinity. Cf. the law of Diocletian recited at Canon LXI of Elvira.

Canon LXXVII. He who leaves the woman legitimately coupled to him and marries another is subjected, according to the sentence of the Lord, to the judgment of adultery. But it has been established by the canons of our Fathers that they weep for one year, hear for two years, be prostrate for three years, in the seventh stand with the faithful, and thus be held worthy of the oblation, if with tears they have done penance.

Canon LXXVIII. The same form is also to be observed toward those who marry two sisters, even at diverse times.

Cf. Canon XXIII.

Canon LXXIX. But those who are mad for their stepmothers are liable to the same canon as those who are mad for their sisters.

Canon LXXX. The Fathers passed over polygamy in silence (in their penitential canons), as utterly bestial and alien from the race of men. But this seems to us to be a sin greater than fornication. Wherefore it is consonant that they be subjected to the canons, namely, so that after they have wept for one year and been prostrate for three, they may thus be received.

Cf. Canons L and IV from these canons of Basil, and Thesis XII, VI and following.