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 (KJV)

Then: Paul restates his belief that men should stay married - adding a twist in focus. He considers the non-believing status of the partner, yet determines the couple should remain married. Most likely this was a detail question which the Corinthians asked. Paul's answer differs from guidelines given to the Israelites of old. When they were led astray by the proliferation of idols and false gods of non-believing partners, the priests directed the people to put away the non-believers. Life changed with Christ whose death marked the way for all believers and not-yet believers to be forgiven. Perhaps that's why Paul says "speak I, not the Lord." Old Testament history lacks words from God on the subject.

Now: Believers who have the gift of the Lord's indwelling spirit know Paul is speaking towards the ultimate purpose that all should be saved and become followers of Christ. They also know their part in his service of going into all the world, teaching the gospel and baptizing many into the kingdom. It is through his spirit that they sometimes need to discern whether to follow Paul's advice or not. For Jesus also gave another principle of not casting pearls before swine. A woman married to an abusive husband - physically, mentally, or emotionally needs the spirit's guidance to know if her ultimate death would help bring the spouse into the kingdom or if the sacrifice of her life would be in vain. The same is true if the abused spouse is the husband. Listening carefully to the Holy Spirit provides answers that can only be known to the Lord.