¹ Do not be mismatched with unbelievers for what fellowship has righteousness with unrighteousness? And what communion has light with darkness? Obviously this and the remaining verses refer to being in a sect calling themselves Christians but which does not believe in the true gospel — the true gospel being that all men in the world through obedience to the light and word within them can receive the cleansing power of God through the ministering grace of the Spirit to: become (not be) a son of God, to be released from the bondage of sin, to have Christ resurrected within them, to be in union with Christ and God, and to enter the kingdom while on earth; furthermore this sinless state is a perfection from sin forever, protected by God from ever falling to sin again.

Even worse is for a sincere seeker of Christ's grace to be yoked in a fellowship with people who don't believe in Christ or even God (i.e., Quakers today, whose membership is freely open to atheists, Buddhists, Hindus, witches, pagans, etc.).

But also, don't enter into business partnerships, marriage, or any joint venture with a person who does not share your faith and hope. But if you are already married to a non-believer, you are to remain married to them as long as they continue to live with you:

To the rest I (not the Lord) say this: If any brother has a wife who does not believe, and she is content to live with him, let him not divorce her.
And if a woman has a husband who does not believe, and he is content to dwell with her, let her not leave him. 1 Cor 7:12-13
But if the unbelieving spouse departs, let him depart. A brother or a sister is not under obligation [to oppose or follow] in such cases. God has called us to peace. 1 Cor 7:12-13,15

_______________________________________