LXXV. The man who has been polluted with his own sister, either on the father's or the mother's side, must not be allowed to enter the house of prayer, until he has given up his iniquitous and unlawful conduct. And, after he has come to a sense of that fearful sin, let him weep for three years standing at the door of the house of prayer, and entreating the people as they go in to prayer that each and all will mercifully offer on his behalf their prayers with earnestness to the Lord. After this let him be received for another period of three years to hearing alone, and while hearing the Scriptures and the instruction, let him be expelled and not be admitted to prayer. Afterwards, if he has asked it with tears and has fallen before the Lord with contrition of heart and great humiliation, let kneeling be accorded to him during other three years. Thus, when he shall have worthily shown the fruits of repentance, let him be received in the tenth year to the prayer of the faithful without oblation; and after standing with the faithful in prayer for two years, then, and not till then, let him be held worthy of the communion of the good thing.
LXXVI. The same rule applies to those who take their own daughters in law.
LXXVII. He who abandons the wife, lawfully united to him, is subject by the sentence of the Lord to the penalty of adultery. But it has been laid down as a canon by our Fathers that such sinners should weep for a year, be hearers for two years, in kneeling for three years, stand with the faithful in the seventh; and thus be deemed worthy of the oblation, if they have repented with tears.
LXXVIII. Let the same rule hold good in the case of those who marry two sisters, although at different times.
LXXIX. Men who rage after their stepmothers are subject to the same canon as those who rage after their sisters.
LXXX. On polygamy the Fathers are silent, as being brutish and altogether inhuman. The sin seems to me worse than fornication. It is therefore reasonable that such sinners should be subject to the canons; namely a year's weeping, three years kneeling and then reception.
LXXXI. During the invasion of the barbarians many men have sworn heathen oaths, tasted things unlawfully offered them in magic temples and so have broken their faith in God. Let regulations be made in the case of these men in accordance with the canons laid down by our Fathers. Those who have endured grievous tortures and have been forced to denial, through inability to sustain the anguish, may be excluded for three years, hearers for two, kneelers for three, and so be received into communion. Those who have abandoned their faith in God, laying hands on the tables of the demons and swearing heathen oaths, without under going great violence, should be excluded for three years, hearers for two. When they have prayed for three years as kneelers, and have stood other three with the faithful in supplication, then let them be received into the communion of the good thing.
LXXXII. As to perjurers, if they have broken their oaths under violent compulsion, they are under lighter penalties and may therefore be received after six years. If they break their faith without compulsion, let them be weepers for two years, hearers for three, pray as kneelers for five, during two be received into the communion of prayer, without oblation, and so at last, after giving proof of due repentance, they shall be restored to the communion of the body of Christ.
LXXXIII. Consulters of soothsayers and they who follow heathen customs, or bring persons into their houses to discover remedies and to effect purification, should fall under the canon of six years. After weeping a year, hearing a year, kneeling for three years and standing with the faithful for a year so let them be received.
LXXXIV. I write all this with a view to testing the fruits of repentance. I do not decide such matters absolutely by time, but I give heed to the manner of penance. If men are in a state in which they find it hard to be weaned from their own ways and choose rather to serve the pleasures of the flesh than to serve the Lord, and refuse to accept the Gospel life, there is no common ground between me and them. In the midst of a disobedient and gainsaying people I have been taught to hear the words “Save your own soul.” Do not then let us consent to perish together with such sinners. Let us fear the awful judgment. Let us keep before our eyes the terrible day of the retribution of the Lord. Let us not consent to perish in other men's sins, for if the terrors of the Lord have not taught us, if so great calamities have not brought us to feel that it is because of our iniquity that the Lord has abandoned us, and given us into the hands of barbarians, that the people have been led captive before our foes and given over to dispersion, because the bearers of Christ's name have dared such deeds; if they have not known nor understood that it is for these reasons that the wrath of God has come upon us, what common ground of argument have I with them?
But we ought to testify to them day and night, alike in public and in private. Let us not consent to be drawn away with them in their wickedness. Let us above all pray that we may do them good, and rescue them from the snare of the evil one. If we cannot do this, let us at all events do our best to save our own souls from everlasting damnation.
Source: Letters (New Advent)