Deuteronomy 13:18

when thou shalt hearken to the voice of the Lord, thy God, to keep all His commandments which I command thee this day, including these, which enjoined the utmost severity upon the false prophets and seducers of every kind, to do that which is right in the eyes of the Lord, thy God. The command of God. as it concerns Christian congregations in our days, plainly states that they must have no fellowship with unbelievers and with false believers. Only in this way is it possible to hinder the corruption and the defection of the people of God. Cf 2 Corinthians 6:14.