1Co 5:12 For what have I to do to judge them also that are without? do not ye judge them that are within?
Christians are to judge other Christians based on their adherence to the Laws of God.
Just so there is no confusion here with Matthew 7:1, where we are commanded not to judge, we need to remember we are commanded to “distinguish” as to whether a believer keeps the Laws of God.
We are not allowed to effect discipline on that person. In that sense God will judge or repay, Rm 12:19.
Ro 12:19 Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord.