Jesus is always the one who surprises. We think we know what piety and devotion are . . . until Jesus tells us to do the opposite.
"Then said he unto the disciples, 'It is impossible but that offences will come: but woe unto him, through whom they come! It were better for him that a millstone were hanged about his neck, and he cast into the sea, than that he should offend one of these little ones. Take heed to yourselves: If thy brother trespass against thee, rebuke him; and if he repent, forgive him" (Luke 17:1-3).
Scandals will come, the Lord says. Nevertheless, the one who brings them is held responsible, and cannot appeal to the "way things are," or to his own valiant efforts to fulfill the Lord's prophecy. Such a man is held responsible in a striking way. If he scandalizes "one of these little ones," it would be better for him to have a millstone tied around his neck and be thrown into the sea. In other words, what is actually going to happen to him is going to be a whole lot worse than that.
So, Jesus says, take heed to yourselves. So, Jesus says, pay attention to yourselves! Now when the Lord tells us to pay attention to ourselves and our own hearts, we all crawl off to pay attention to no one's sins but our own. True piety demands it. But what does the Lord actually say? He says that we should pay attention to ourselves, to wit, we should rebuke the guy who sins against us. Huh, say the faithful. Didn't see that one coming.
In the previous chapter, the Lord says that the children of this world are, in their rough and ready way, wiser than the children of light (Luke 16:8). Put another way, there are certain situations where you can trust worldlings better than you can trust saintlings. This, I think, is one of those areas. The devout frequently mistrust themselves to a paralyzing extent. We obey part of Gal. 6:1, considering ourselves lest we also be tempted right out to the utter frozen limit.
In part we do this because when we look around in the body of Christ, we do see those who have a "ministry of discernment and rebuke," and we don't want to be like that. We have seen or heard of letters to the pastor that begin with "I write this with a heavy heart," and no wise person wants to be that guy. So we back off entirely. But this has exactly the same effect as leaving wine entirely to the stewardship of drunks. Nobody ever sees it done right -- because nobody ever does it the way Jesus says to.
We have left all the rebuking to those who have an ax to grind. We have delegated rebuking to those who have no idea of what is going on. We appoint the whole task to the confused, and are surprised at all the resultant confusion.
So then, pay close attention to your own heart. This means telling your brother what he has been doing. You should make a point of telling him to quit it.