1 Corinthians 7:12
But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away.
The instruction concerns a marriage entered before conversion, where one partner has come to believe and the other has not: if the unbelieving wife is content to live with the believing husband, he is not to send her away but is to continue in the marriage, and the believing partner is to remain unless the unbeliever chooses to depart. Mere unbelief does not dissolve the bond, and neither opposition, nor disagreement over keeping the Sabbath, nor the hardship and persecution that may follow obedience, gives the believer a right to divorce.
The one who remains is to be kind, faithful and true, showing love and meekness, placing loyalty to God above every earthly tie even when faithfulness brings trial or persecution, looking to God for strength and wisdom, and acting from firm, pure principle rather than from impulse or lust. Such steadfast faithfulness can exert a sanctifying influence upon the unbelieving partner and may win that partner, so the believer should endeavor, as far as possible, to lead the spouse into the light while yielding nothing of convictions of duty. What is taught of the believing wife holds likewise for a believing husband or for a believing child in a divided household.
This counsel for marriages already formed gives no permission to enter a new marriage deliberately with an unbeliever, for unions knowingly made between believers and unbelievers are contrary to God's word. To part a husband and wife is a grave act that must rest on Scriptural ground: a husband who did not put away his wife but was left by her is not to be separated from her, and where a wife has separated herself by divorce and married another man, and her former husband has entered a new marriage only after that divorce, the later union is not to be broken up.