Matthew 18:15-17, 20

“If your brother sins, go and show him his fault in private; if he listens and pays attention to you, you have won back your brother. But if he does not listen, take along with you one or two others, so that EVERY WORD MAY BE CONFIRMED BY THE TESTIMONY OF TWO OR THREE WITNESSES. If he pays no attention to them [refusing to listen and obey], tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile (unbeliever) and a tax collector. [Lev 19:17; Deut 19:15]

For where two or three are gathered in My name [meeting together as My followers], I am there among them.” [Ex 3:14]