Marriage is a lifelong commitment.
In GOD's design, marriage is a lifelong commitment that only ends when one or both spouses dies.
When a groom and a bride vow to become husband and wife, GOD joins the two together as one. This goes back to the beginning where GOD created Eve and gave her to Adam:
18 And the Lord God said, “It is not good for man to be alone. I will make him a helper comparable to him.” 19 Also, God formed out of the ground all the wild animals of the field and all the birds of heaven, and brought them to Adam to see what he would call them. Thus whatever Adam called each living creature, that was its name. 20 So Adam gave names to all the cattle, to all the birds of heaven, and to all the wild animals of the field. But for Adam there was not found a helper comparable to him. 21 Thus God brought a trance upon Adam, and he slept; and He took one of his ribs, and filled up the flesh in its place. 22 Then the Lord God built the rib He took from Adam into a woman, and brought her to him. 23 So Adam said, “This is now bone of my bones, and flesh of my flesh. She shall be called Woman, because she was taken out of Man. 24 For this reason a man shall leave his father and mother and be joined to his wife; and the two shall become one flesh.” 25 Now the two were naked, both Adam and his wife, and were not ashamed.
(Genesis 2:18-25 OSB)
Vows are serious to GOD. GOD expects us to keep our word. When we promise, He holds us to that promise.
In a perfect world, every couple that committed to be married and to keep themselves pure for each other would follow through with that commitment.
We don't live in a perfect world.
When a couple is joined together as one by GOD, only GOD can decide to disjoin them. Don't miss that because it's critical.
This means that, when a husband and wife divorce, if GOD does not disjoin them in His eyes, the first time either of them has sex with someone else—even if they have chosen to marry another person and are legally married—in GOD's eyes, that person committed adultery because He saw them as still being joined to their original spouse.
This is why Jesus said that the man who divorces his wife (except for sexual immorality) causes her to commit adultery. Bearing in mind that Jesus lived under the Mosaic Law (which Jesus fulfilled), consider Jesus' teaching:
2 Some Pharisees came and tested him by asking, “Is it lawful for a man to divorce his wife?”
3 “What did Moses command you?” he replied.
4 They said, “Moses permitted a man to write a certificate of divorce and send her away.”
5 “It was because your hearts were hard that Moses wrote you this law,” Jesus replied. 6 “But at the beginning of creation God ‘made them male and female.’ 7 ‘For this reason a man will leave his father and mother and be united to his wife, 8 and the two will become one flesh.’ So they are no longer two, but one flesh. 9 Therefore what God has joined together, let no one separate.”
10 When they were in the house again, the disciples asked Jesus about this. 11 He answered, “Anyone who divorces his wife and marries another woman commits adultery against her. 12 And if she divorces her husband and marries another man, she commits adultery.”
(Mark 10:2-12 NIV)
While the Mosaic Law is not binding on Christians (thank you Jesus!), note that Jesus' teaching here is based on truths that pre-date the Mosaic Law (and therefore remain forever).
For as long as the earth shall last, GOD's principle is that He makes the two one flesh. Only He can separate that one flesh.
GOD has provided two allowances whereby a husband or wife may know that He has released them from their "one flesh" with their original spouse:
- Death of the other spouse.
- Adultery of the other spouse.
In the case that one's spouse commits adultery, GOD then provides that the faithful spouse may divorce because their spouse broke their wedding vow (see Matthew 19:9).
Sin is always associated with divorce.
- Either one/both spouse(s) separates (divorces) from the oneness GOD established (and maintains), or ...
- A spouse commits adultery.
Marriages can only be disjoined by GOD. In GOD's eyes, marriage is a lifelong commitment.