11 O ye Corinthians, our mouth is open unto you, our heart is enlarged. 12 Ye are not straitened in us, but ye are straitened in your own bowels. 13 Now for a recompence in the same, (I speak as unto my children,) be ye also enlarged. 14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? 15 And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? 16 And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people. 17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you. 18 And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.
The principle of separation from error will not permit true churches to join forces with other religions.