JESUS TO THE JEWS:
Mark 10:2-12 NASB
2 Some Pharisees came up to Jesus, testing Him, and began to question Him whether it was lawful for a man to divorce a wife. 3 And He answered and said to them, “What did Moses command you?” 4 They said, “Moses permitted a man TO WRITE A CERTIFICATE OF DIVORCE AND SEND her AWAY.” 5 But Jesus said to them, “Because of your hardness of heart he wrote you this commandment. 6 But from the beginning of creation, God MADE THEM MALE AND FEMALE. 7 FOR THIS REASON A MAN SHALL LEAVE HIS FATHER AND MOTHER, 8 AND THE TWO SHALL BECOME ONE FLESH; so they are no longer two, but one flesh. 9 What therefore God has joined together, let no man separate.”
10 In the house the disciples began questioning Him about this again. 11 And He *said to them, “Whoever [fn]divorces his wife and marries another woman commits adultery against her; 12 and if she herself divorces her husband and marries another man, she is committing adultery.”
[Jesus treats the marriage of two 'Jews' as legal and binding because only a marriage within the covenent of God (old or new) is a real marriage covenent with God and your spouse.]
PAUL TO THE GENTILES:
1 Corinthians 7:10-16 NASB
10 But to the married I give instructions, not I, but the Lord, that the wife should not leave her husband 11 (but if she does leave, she must remain unmarried, or else be reconciled to her husband), and that the husband should not divorce his wife.
[Paul treats the marriage of two Christian converts (two former pagans) as legal and binding ... because only a marriage within the covenent of God (old or new) is a real marriage covenent with God and your spouse.]
12 But to the rest I say, not the Lord, that if any brother has a wife who is an unbeliever, and she consents to live with him, he must not divorce her. 13 And a woman who has an unbelieving husband, and he consents to live with her, she must not send her husband away. 14 For the unbelieving husband is sanctified through his wife, and the unbelieving wife is sanctified through her believing husband; for otherwise your children are unclean, but now they are holy. 15 Yet if the unbelieving one leaves, let him leave; the brother or the sister is not under bondage in such cases, but God has called us to peace. 16 For how do you know, O wife, whether you will save your husband? Or how do you know, O husband, whether you will save your wife?
[Paul treats the 'marriage' between a believer and an unbeliever as an oportunity for evangelism. If the unbeliever walks away, then let them go. The "brother or the sister is not under bondage in such cases". It sort of reminds me of this: Mark 6:11 NASB "Any place that does not receive you or listen to you, as you go out from there, shake the dust off the soles of your feet for a testimony against them." ]