But if the unbelieving partner separates, let it be so; in such a case the brother or sister is not bound. It is to peace that God has called you. Wife, for all you know, you might save your husband. Husband, for all you know, you might save your wife. (1Corinthians 7:15-16)

The translator has sometimes distinquished between believers and non-believers. Here (s)he is much closer to the literal Greek. The original distinction is adelphos and apistos.

Adelphos is to share the same womb, to be brothers and sisters. Believers share a common origin and identity in Christ. When we accept Christ we are brought into relationship with all who have accepted Christ.

Apistos is unfaithful, unconvinced, unbelieving. Paul perceives that the exercise of choice is essential to authentic relationship. None are bound. All are free. The relationship with Christ is available to all, but the choice is left to each.

Individual choice is so crucial to Paul's understanding of God's intention that the right of separation is affirmed. For Paul such separation is an illusion - a shadow reality - but individual choice must be honored even in error.

We are called to peace - eirene - to be joined in harmony. The call of peace includes respect for bad, foolish, and even delusional choices.

I do not know how to resolve this conclusion with the condemnation and exclusion of the man living with his father's widow. Paul moves from principled acceptance to particular condemnation for reasons that are not - yet - clear to me.