Nowhere in the Old or New Testaments does God say that people who have remarried need to divorce their new spouse and remarry their first spouse. In fact, under the Law of Moses, the closest scenario strictly forbids remarrying the first spouse:
{1} When a man takes a wife and marries her, if then she finds no favor in his eyes because he has found some indecency in her, and he writes her a certificate of divorce and puts it in her hand and sends her out of his house, and she departs out of his house, {2} and if she goes and becomes another man’s wife, {3} and the latter man hates her and writes her a certificate of divorce and puts it in her hand and sends her out of his house, or if the latter man dies, who took her to be his wife, {4} then her former husband, who sent her away, may not take her again to be his wife, after she has been defiled, for that is an abomination before the Lord. And you shall not bring sin upon the land that the Lord your God is giving you for an inheritance. (Deut. 24:1-4)
Christians are not under the Law of Moses, but to say God ‘requires‘ people to divorce in order to remarry their first spouse is at least unusual, given God’s instructions above.
if a person comes to christ and is saved for a first time their past sins are forgiven, including divorce.
If christians divorce (except for fornication or a compunctual cheating spouse) they are not bound under law for truly we are free from law and dead works. BUT, let them pray about it and discover what God wants, for transgressed they have; perhaps for singleness sake, not all people need to be married.
Gods perfect will in unstable marriages and divorces is reconciliation and restitution.