When you see sin or damaging behavior in someone else's life, how do you know when to keep quiet and when to speak with them?
The first principle that Paul lays down for us is, "Who are we to judge those who are outside? It is those in the church that we are to judge." So the first answer is, I'm watching sinful destructive behavior all day in the world. Television, movies, YouTube, on the street, in advertising, people are destroying themselves all day long—neighbors and people all around us.
You don't go to everybody. You are not called to spend 18 hours a day walking up to people saying, "Don't smoke!" or, "Don't drink!" or, "Don't swear!" or, "Don't hit your wife!" or, "Don't fail to discipline your children!"
That's not our job. We preach the gospel to the world, and as occasion arises we might link some destructive behavior to the gospel as a way out.
In the church the question becomes more urgent. In the church, the answer to the question is going to hang on criteria like, how serious is the sin? If it is really serious, immediately urgent, and you know that the person is a part of the church—even if you don't know them personally—you might go and do Galatians 6:2. "If you find a brother taken in a fault, restore such a one in a spirit of meekness lest you too be tempted."
So the mindset is, I've got a log hanging out of my eye. Now compared to a log, this person's behavior is a speck. Or even if it is a log, I've got my own log. I can't go to him with a log hanging out of my eye because the log will hit him on the head and do more damage than if I took my log out first.
So my first job is to take out my own log. Then I see clearly, Jesus says, to take the speck out of my brother's eye. So I've become a successful eye surgeon of the sin speck in my brother's eye by getting the log out of my own eye.
So the criteria is, how serious is the sin, and am I spiritually equipped. And you go in there and try to speak in a way that wouldn't feel condemning—at least at first. You may have to get tough later, but at first you want to win them. You want to create a bubble of grace in which they feel some hope that even though this is sin, they are loved and accepted.
Another criterion would be, how close is your relationship? Are there other people in this person's life? If I saw somebody in your small group doing something, and you are the leader of that little group, I might ask you, "Are you concerned about this kind of thing? You might watch out for it in your group." Because I would rather have someone they know pursue them this way than somebody that has less of a relationship with them.
My final answer would be, have spiritual discernment and spiritual wisdom for the moment about whether this is an auspicious helpful time to talk, or whether another angle would be better.