If a man and woman are married by civil law and get divorced, and neither ever remarry, and they start dating and decide to live together, are they married in God’s eyes? And if a man and woman who have never been married by civil law, but they live together till one of them dies, are they living and dying in sin?
1 Cor. 7:10-11 says
A wife must not separate from her husband. But if she does, she must remain unmarried or else be reconciled to her husband. And a husband must not divorce his wife.
This is the Biblical standard. Deviation from this standard is a sin. If a husband and wife separate and later get back together they’ve met the standard of 1 Cor. 7:10-11. Legal marriage ceremonies, whether civil or religious, are man made in nature and are not required to meet the Biblical standard. In the Lord’s eyes a believing couple is married when the intent of their hearts is to be considered as such before Him.
A number of states in the US recognize unions like this as common-law marriages as do many other countries around the world. (Of course civil authorities don’t require the couple to be believers like the Bible does).