Jesus said in John 12:47-48 that his words will judge us in the day of judgment:
“If anyone hears my words but does not keep them, I do not judge that person. For I did not come to judge the world, but to save the world.
There is a judge for the one who rejects me and does not accept my words; the very words I have spoken will condemn them at the last day.
And that heaven and earth will pass away but his words will not (Matt 24:35). And it was in this context (Matt 5:18) that Jesus gave his prohibition on remarriage (Matt 5:32) also warning that not many would receive this word because it would imply forced celibacy if the other spouse left (Matthew 19:11-12).
There are a lot of testimonies where this rhema word was active in divorced and remarried couples where verses like Luke 16:18 would not leave them alone "Anyone who divorces his wife and marries another woman commits adultery, and the man who marries a divorced woman commits adultery." and they were driven to repent of remarriage adultery and make efforts to return to their covenant spouse. (Source:
https://cadz.net/olga.html)
One reason remarriage while the first spouse is alive is not allowed is because the marriage covenant is sacred. The common wedding vow today contains the words "til death do us part" not "til divorce do us part". A remarriage vow would contradict the first wedding vow while the first is still active and hence would be invalid.
God takes covenants seriously just like he did with Joshua and the Gibeonites. When Saul killed the Gibeonites, God would not heal the land until seven of Sauls sons were hanged. It was not enough for the Israelites to feel sorry (2 Samuel 21).
In Malachi 2:14, God speaks to Israelites who divorced and remarried as if they were still married to the their original wife, referring to her as the "wife of your covenant". If this is the case then someone in a second marriage would be living in adultery in God's eyes since God still views the husband as married to their first wife.
Malachi 2:14-16 [Amiplified]
But you say, “Why [does He reject it]?” Because the*Lord*has been a witness between you and the wife of your youth, against whom you have dealt treacherously.
Yet she is your marriage companion and the wife of your covenant [made by your vows]. But not one has done so who has a remnant of the Spirit. And what did that one do while seeking a godly offspring? Take heed then to your spirit, and let no one deal treacherously against the wife of your youth. “For I hate*divorce,” says the*Lord, the God of Israel, “and him who covers his garment with wrong*and*violence,” says the*Lord*of hosts. “Therefore keep watch on your spirit, so that you do not deal treacherously [with your wife].”
Romans 7:2-3
For a married woman is bound by law to her husband while he lives, but if her husband dies she is released from the law of marriage.
Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is free from that law, and if she marries another man she is not an adulteress.
Dr. Joseph Webb writes in more detail on the seriousness of the marriage covenant in his book "Til Death to us Part?" and in this video series:
https://www.youtube.com/watch?v=AeXF...OEYPBH5LeSXL6G