Whose business is it to judge?

“What business is it of mine to judge those outside the church? Are you not to judge those inside? God will judge those outside. ‘Expel the wicked man from among you.’” 1 Corinthians 5:12-13
Passing judgement is a topic that most people have a strong objection to. If they are confronted about some sin in their life, they might say, ‘But the Bible says, ‘Judge not, let you be judged’!’ (Matt. 7:1-2). But if that’s what Jesus really meant, why did He also give us a procedure for how to confront a brother about his sin, in Matt. 18:15-17?
In today’s verse, Paul makes a distinction in how we should and shouldn’t judge people. The distinction is quite clear: are they believers, in the church, or are the unbelievers, outside the church?
It’s not up to us to pass judgement on the sins of unbelievers. We are to leave that to God, knowing that He will bring judgement at the last day, and His judgement will be fair (John 16:8, Rom. 2:5, etc.). Instead, we are to share with them the gospel. This is what they need, more than giving up some sin or other. As the saying goes, you don’t clean the fish first – you catch it first, then you clean it.
However, we do have a Scriptural mandate for judging the sins of believers. In this particular instance, Paul was calling for them to punish a man who had taken his stepmother by putting him out of their fellowship. The reason we can do this is because believers ought to know better than to continue in sin.
Note that this judgement applies only to outward sins. We must not, because we cannot, judge somebody’s motives, because we can’t see their heart – only God can see what’s in their heart. But we can judge someone’s outward actions – and we must, for the health of the body of Christ.