Rebuke

The word Rebuke is active second person singular and means to "charge some one as being blamable, strictly assess a penalty, and to reprove, warn, strongly admonish

In the Bible there are two categories of who gets Rebuked:

1. Demons out of people, and Satan (both the lord and Michael)
2. Brothers (fellow believers) who sin

Some have said that mercy and forgiveness are better than condemnation. Of course we would forgive any sin if repented.  However, it is not merciful to let a brother persist in his error, which could lead him to damnation. We are supposed to bring our brothers sin to him, so that he will regain his faith and that God will have mercy on him (Matthew 18:15, Galatians 6:1). It is a Biblical mandate to rebuke our erring brother (Galatians 1:6-9, Galatians 5:7-10, Titus 1:9-13, 2 Timothy 2:16-17, 2 Timothy 4:2-5). Remember also, that it is a sin to not do the good we are supposed to (James 4:17). Lastly, CFW Walther's 8th thesis in Law and Gospel is that "You are not rightly distinguishing Law and Gospel in the Word of God if you preach... the Gospel to those who are living securely in their sins."

Romans 16:17I appeal to you, brothers, to watch out for those who cause divisions and create obstacles contrary to the doctrine that you have been taught; avoid them.

2 Timothy 3:16-17

All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work.

2 Timothy 4:2-5

preach the word; be ready in season and out of season; reprove, rebuke, and exhort, with complete patience and teaching. For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions, and will turn away from listening to the truth and wander off into myths. As for you, always be sober-minded, endure suffering, do the work of an evangelist, fulfill your ministry

Titus 1:9-11,13

He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it. For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. They must be silenced, since they are upsetting whole families by teaching for shameful gain what they ought not to teach…. Therefore rebuke them sharply, that they may be sound in the faith,

Galatians 1:6-9

I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.

Galatians 5:7-10

You were running well. Who hindered you from obeying the truth? This persuasion is not from him who calls you. A little leaven leavens the whole lump.  I have confidence in the Lord that you will take no other view, and the one who is troubling you will bear the penalty, whoever he is.