Is it a sin if two spouses have divorced (the marriage certificate has been canceled) and continue to live together as husband and wife, as they did before the cancellation of the certificate of marriage?
Yes, these spouses sin. They commit adultery. Sexual relations are permitted by God only in the covenant of marriage. Divorce is a bad thing and it is the breaking of the covenant of marriage. Since the covenant was broken, any sexual relationship is outside of it, and so, it is adultery, and God says:
Marriage is to be held in honor among all, and the marriage bed is to be undefiled;for fornicators and adulterers God will judge. (Hebrews 13:4)(NASB)
The very fact that they came to divorce, defiled their marriage, but now, they totally dishonor this covenant. My advice to them is to repent, to become children of God, to stop any sexual relationship until they restore their marriage covenant and then held their marriage in honor.
Translated by Felicia Rotaru