The law of marriage is a vow the binds the couple as long as they both shall live. Romans 7:1-3 “Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth? For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband. So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man.”
In OT times, someone who made such a vow did so before witnesses at the gate of the city. We have the experience of Ruth and Boaz: Ruth 4:1-8 “Then went Boaz up to the gate, and sat him down there: and, behold, the kinsman of whom Boaz spake came by; unto whom he said, Ho, such a one! turn aside, sit down here. And he turned aside, and sat down. And he took ten men of the elders of the city, and said, Sit ye down here. And they sat down. And he said unto the kinsman, Naomi, that is come again out of the country of Moab, selleth a parcel of land, which was our brother Elimelech’s: And I thought to advertise thee, saying, Buy it before the inhabitants, and before the elders of my people. If thou wilt redeem it, redeem it: but if thou wilt not redeem it, then tell me, that I may know: for there is none to redeem it beside thee; and I am after thee. And he said, I will redeem it. Then said Boaz, What day thou buyest the field of the hand of Naomi, thou must buy it also of Ruth the Moabitess, the wife of the dead, to raise up the name of the dead upon his inheritance. And the kinsman said, I cannot redeem it for myself, lest I mar mine own inheritance: redeem thou my right to thyself; for I cannot redeem it. Now this was the manner in former time in Israel concerning redeeming and concerning changing, for to confirm all things; a man plucked off his shoe, and gave it to his neighbour: and this was a testimony in Israel. Therefore the kinsman said unto Boaz, Buy it for thee. So he drew off his shoe.” That shoe was a testimony in Israel that they had entered into such an arrangement. And she became his wife. Ruth 4:13 “So Boaz took Ruth, and she was his wife: and when he went in unto her, the LORD gave her conception, and she bare a son.”
Now we do not carry the other person’s shoe but we do have other methods that show legality. 1 Corinthians 14:40 “Let all things be done decently and in order.” Part of this orderliness is to observe the laws of the land as today they are acting the part of civil government as those that sat at the gate. Romans 13:3-4 “For rulers are not a terror to good works, but to the evil. Wilt thou then not be afraid of the power? do that which is good, and thou shalt have praise of the same: For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil.” Therefore, those who enter the marriage relation, they need to safeguard it with legal documents so that if someone wants to tear apart the marriage vow, they have the civil government to fear as well as the spiritual.