"Don’t team up with those who are unbelievers. How can righteousness be a partner with wickedness? How can light live with darkness? What harmony can there be between Christ and the devil? How can a believer be a partner with an unbeliever? And what union can there be between God’s temple and idols? For we are the temple of the living God. As God said: “I will live in them and walk among them. I will be their God, and they will be my people. Therefore, come out from among unbelievers, and separate yourselves from them, says the LORD. Don’t touch their filthy things, and I will welcome you. And I will be your Father, and you will be my sons and daughters, says the LORD Almighty.” 2 Corinthians 6:14-18
"You shall not intermarry with them, giving your daughters to their sons or taking their daughters for your sons, for they would turn away your sons from following me, to serve other gods. Then the anger of the Lord would be kindled against you, and he would destroy you quickly." Deuteronomy 7:3-4
However, if one is already married to an unbeliever, this is what the Bible says about the subject:
"To the rest I say that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy. But if the unbelieving partner separates, let it be so. In such cases the brother or sister is under no obligation to remain with them. God has called you to peace. For how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?" 1 Corinthians 7:12-16
This does not mean you cannot have non-Christian friends or friends from other religions, It just means you should not partner with them or marry them. Of course it also means that your tribe as a believer is the tribe of the Christian brotherhood. We are brothers and sisters in Christ and our primary source of camaraderie ought to be other believers.