'Therefore come out from them and be separate, says the Lord.
Touch no unclean thing, and I will receive you.
I will be a Father to you, and you will be my sons and daughters,
says the Lord Almighty."
Do Not Be Yoked With Unbelievers
Paul urges believers not to form binding relationships with nonbelievers, because this might weaken their Christian commitment, integrity, or standards. It would be a mismatch! He was not saying to isolate ones self from nonbelievers, but to be active in their witness for Christ. Rather he says not to lock yourself in a relationship that would cause you to compromise your Christian principals.