Nay, you do wrong, and defraud, and that your brethren.
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Paul is rebuking the people whose wrongful behavior has started the quarrels. Not only are they liable to be charged for the fraud which they have committed; they also share in the fault of those who, compelled by their injurious or fraudulent actions, call upon unbelievers to pass judgment. Commentary on Paul’s Epistles. ...
Defraud. Your brethren. That is, you still make yourselves much more guilty by the injustices done to one another: for the unjust, and all they who are guilty of such crimes as I have mentioned, shall not possess the kingdom of God. And some of you were guilty of part of them, which have been washed off by your conversion, and your baptism, when you were justified. (Witham)
And such some of you were. It is probable that this was added by the apostle, to soften his preceding words, lest he might seem to accuse all the Corinthians of each of these sins, and he likewise adds, such indeed you were, but now you are washed (Estius; St. Thomas Aquinas) ...
Again, it is a twofold crime, perhaps even threefold or fourfold. One, not to know how to bear being wronged. Another, actually to do wrong. A third, to commit the settlement of these matters even unto the unjust. And yet a fourth, that it should be so done to a brother. For men's offenses are not judged by the same rule, when they are committed against any chance person, and towards one's own member. For it must be a greater degree of recklessness to venture upon that. In the other case, the nature of the thing is alone trampled on; but in this, the quality of the person also.
8. Having thus, you see, abashed them from arguments on general principles, and before that, from the rewards proposed ; he shuts up the exhortation with a threat, making his speech more peremptory, and saying thus, 1 Corinthians 6:9 Do you not know that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of the...
Here there may be as many as four crimes involved. The first is not knowing how to bear being wronged. The second is to do wrong. The third is to reserve settlement of the matter to the unrighteous. And the fourth is that this kind of thing is being done to a fellow believer.