Translation
However, if the unbeliever separates himself, let him separate himself; the brother or the sister is not under bondage in such. However, THEOS has called you to peace.
Paraphrase
However, if the spouse that is not a believer wants a separation or a divorce, let the separation or divorce happen; the brother in Christ or the sister in Christ is not obliged to remain in such a relationship. Instead, THE CREATOR AND OWNER OF ALL THINGS has called us to live in peace with those around us.
NOT UNDER BONDAGE BUT IN PEACE
The believing spouse is not obliged by God to forcibly keep the marriage relationship together. If force were used, it would only result in keeping it together on the outside, for the sake of how it appears to others, but the relationship itself would be totally destroyed. God does not seek that we live in bondage, rather He wants our relationships to be characterized by peace, which meant “wholesomeness, wellbeing, healthy, completeness, properly joined together, oneness, harmony, tranquility, rest, security, safety.”