One could consider that it is not a sin for a married couple to have intercourse, not only for the sake of procreating children -- which is the good of marriage -- but also for the sake of the carnal pleasure involved. Thus, those whose self-control is weak could avoid fornication, or adultery, and other kinds of impurity too shameful to name, into which their lust might drag them through Satan's tempting. Therefore one could, as I said, consider this not a sin, had the apostle not added, "But I say this as a concession, not as a rule." Who, then, denies that it is a sin when he agrees that apostolic authority for doing it is given only by "concession"?
Another such case is seen where he says, "Dare any of you, having a case against another, bring it to be judged before the unrighteous and not the saints?"  And a bit later: "If, therefore, you have cases concerning worldly things," he says, "you appoint those who are contemptible in the Church's eyes. I say this to shame you. Can it be that there is not a wise man among you, who could judge between his brethren? But brother goes to law with brother, and that in the presence of unbelievers."  And here it might be thought that it was not a sin to bring suit against a brother, and that the only sin consisted in wishing it judged outside the Church, if the apostle had not added immediately, "Now therefore the whole fault among you is that you have lawsuits with one another."  Then, lest someone excuse himself on this point by saying that he had a just cause and was suffering injustice which he wished removed by judicial sentence, the apostle directly resists such thoughts and excuses by saying: "Why not rather suffer iniquity? Why not rather be defrauded?"  Thus we are brought back to that saying of the Lord: "If anyone would take your tunic and contend in court with you, let go your cloak also."  And in another place: "If a man takes away your goods, seek them not back."  Thus, he forbids his own to go to court with other men in secular suits. And it is because of this teaching that the apostle says that this kind of action is "a fault." Still, when he allows such suits to be decided in the Church, brothers judging brothers, yet sternly forbids such a thing outside the Church, it is clear that some concession is being made here for the infirmities of the weak.
Because of these and similar sins -- and of others even less than these, such as offenses in words and thoughts -- and because, as the apostle James confesses, "we all offend in many things,"  it behooves us to pray to the Lord daily and often, and say, "Forgive us our debts," and not lie about what follows this petition, "As we also forgive our debtors."
79. There are, however, some sins that could be deemed quite trifling if the Scriptures did not show that they are more serious than we think. For who would suppose that one saying to his brother, "You fool," is "in danger of hell-fire," if the Truth had not said it? Still, for the hurt he immediately supplied a medicine, adding the precept of brotherly reconciliation: "If, therefore, you are offering a gift at the altar, and remember there that your brother has something against you,"  etc.
Or who would think how great a sin it is to observe days and months and years and seasons -- as those people do who will or will not begin projects on certain days or in certain months or years, because they follow vain human doctrines and suppose that various seasons are lucky or unlucky -- if we did not infer the magnitude of this evil from the apostle's fear, in saying to such men, "I fear for you, lest perhaps I have labored among you in vain"  ?
80. To this one might add those sins, however grave and terrible, which, when they come to be habitual, are then believed to be trivial or no sins at all. And so far does this go that such sins are not only not kept secret, but are even proclaimed and published abroad -- cases of which it is written, "The sinner is praised in the desires of his soul; and he that works iniquity is blessed." 
In the divine books such iniquity is called a "cry" (clamor). You have such a usage in the prophet Isaiah's reference to the evil vineyard: "I looked that he should perform justice, yet he did iniquity; not justice but a cry."  So also is that passage in Genesis: "The cry of Sodom and Gomorrah is multiplied,"  for among these people such crimes were not only unpunished, but were openly committed, as if sanctioned by law.
So also in our times so many evils, even if not like those [of old], have come to be public customs that we not only do not dare excommunicate a layman; we do not dare degrade a clergyman for them. Thus, several years ago, when I was expounding the Epistle to the Galatians, where the apostle says, "I fear for you, lest perchance I have labored in vain among you," I was moved to exclaim: "Woe to the sins of men! We shrink from them only when we are not accustomed to them. As for those sins to which we are accustomed -- although the blood of the Son of God was shed to wash them away -- although they are so great that the Kingdom of God is wholly closed to them, yet, living with them often we come to tolerate them, and, tolerating them, we even practice some of them! But grant, O Lord, that we do not practice any of them which we could prohibit!" I shall someday know whether immoderate indignation moved me here to speak rashly.
 I:Cor. 7:5 (mixed text).  I:Cor. 6:1.  I:Cor. 6:4-6.  I:Cor. 6:7a.  I:Cor. 6:7b.  Matt. 5:40.  Luke 6:30.  James 3:2 (Vulgate).  Matt. 5:22, 23.  Gal. 4:11 (Vulgate).  Ps. 10:3((Vulgate).  Isa. 5:7 (LXX).  Gen. 18:20 (Vulgate with one change).
 I:Cor. 6:1.
 I:Cor. 6:4-6.
 I:Cor. 6:7a.
 I:Cor. 6:7b.
 Matt. 5:40.
 Luke 6:30.
 James 3:2 (Vulgate).
 Matt. 5:22, 23.
 Gal. 4:11 (Vulgate).
 Ps. 10:3((Vulgate).
 Isa. 5:7 (LXX).
 Gen. 18:20 (Vulgate with one change).