Charles Spurgeon on church membership:
Some people say, “We belong to such-and-such a church, but we don’t approve of its teaching or its practice.”
What! You belong to it, yet you do not approve of its principles? Out of your own mouth you are condemned. If I unite with a church whose creed I do not believe, I am guilty of my own share in all the error that is there. It is no use for me to say, “I am trying to undo the mischief.” I have no business to be there.
2Corinthians 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?