After becoming a Christian, a person who marries an unbeliever, sins against God. Except if they were already married, before one of them became a Christian.
Malachi 2:11-12
Judah hath dealt treacherously, and an abomination is committed in Israel and in Jerusalem; for Judah hath profaned the holiness of the Lord which he loved, and hath married the daughter of a strange god.
The Lord will cut off the man that doeth this, the master and the scholar, out of the tabernacles of Jacob, and him that offereth an offering unto the Lord of hosts.
Do not dismiss the book of Malachi as Old Testament irrelevance, if you use Malachi 3:8 to justify taking tithes or Malachi 2:16 as a blanket ban for divorce. If you claim Malachi 2:11-12, only applies to Jews, is this not racist to imply that only Jews can be lead astray with interfaith marriages?
Ezra 10:10
And Ezra the priest stood up, and said unto them, Ye have transgressed, and have taken strange wives, to increase the trespass of Israel.
Ezra 10:11
Now therefore make confession unto the LORD God of your fathers, and do his pleasure: and separate yourselves from the people of the land, and from the strange wives.
Nehemiah 13:23
In those days also saw I Jews that had married wives of Ashdod, of Ammon, and of Moab.
Nehemiah 13:25
And I contended with them, and cursed them, and smote certain of them, and plucked off their hair, and made them swear by God, saying, Ye shall not give your daughters unto their sons, nor take their daughters unto your sons, or for yourselves.
Nehemiah 13:26
Did not Solomon king of Israel sin by these things? yet among many nations was there no king like him, who was beloved of his God, and God made him king over all Israel: nevertheless even him did outlandish women cause to sin.
Nehemiah 13:27
Shall we then hearken unto you to do all this great evil, to transgress against our God in marrying strange wives?
Nehemiah 13:26, identifies that marrying an unbeliever can cause a follower of God to be lead astray. Are you better than Jews like Solomon, if you think you will not be negatively influenced by an unbelieving spouse? Gentiles and Jews in Christ, should not marry an unbeliever, to protect them from the consequences of interfaith sin.
II Corinthians 6:14
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with
darkness?
Some have dismissed II Corinthians 6:14, as irrelevant to marriage. If God does not want you to have fellowship with unbelievers, how much worse if you marry them? Though II Corinthians 6:14 is not about marriage, the scope of this verse obviously applies to marriage.
I Corinthians 7:39
The wife is bound by the law as long as her husband liveth; but if her husband be dead, she is at liberty to be married to whom she will; only in the Lord.
In the New Testament at 1 Corinthians 7:39, Paul says that a widow can marry again, “only in the Lord.” Paul was casually referring back to the law against marrying unbelievers. Those who are ridiculously legalistic, might say that Paul only places this restriction on widows. Where is the sense in that? Why are widows singled out for not being allowed to marry unbelievers, whereas previously it applied to all of the elect? After God appalled marriages between unbelievers and the elect as an “abomination”, now He suddenly changes His mind and says that anyone can marry unbelievers except widows? Paul was obviously avoiding heresy from his words, “at liberty to be married to whom she will” by limiting the scope of that liberty to Christians.
God does not change
The character of God does not change. God does not need to repeat sins in the New Testament for them to remain forbidden. Else bestiality would now be fine. Jesus said in Mathew 5:17, that He came not, “to destroy the law … but to fulfill.” Because God referred strongly to marriages between the elect and unbelievers as an “abomination”, there would need to be a clear explicit law, to claim that God changed His mind and now supports mixed religious marriages. Common sense construes that unless a sin is explicitly legalized in the New Testament, it must still be considered as forbidden. Making an assumption to the contrary is absurd. Scripture should not even be interpreted as requiring a believer to marry an unbeliever. Do not assume marrying unbelievers is okay by reason of Esther, because she was forced into marriage with a divorced heathen, it was not her fault. The clear statutes and precedence from the Word of God, reveal that the character of God does not change.
When one spouse becomes a Christian
God wants new converts that were already married to unbelievers, to remain together, unless the unbeliever departs, according to 1 Corinthians 7:12-15. This is different from Deuteronomy 21:10-14, where the person was already a follower of God, when they chose to marry an unbeliever under the special circumstance of having captured her in warfare. It could be assumed that our loving God allowed for this exception clause, because a woman captured in warfare even in our present day, might be raped, mistreated, depraved, or possibly even killed. By marrying a man of God from the victorious enemy country, she could avoid the evils mentioned. If a Christian does marry an unbeliever, there is precedent that such marriages are valid. Esther and the divorced King Xerxes are one example. Samson and Solomon with their rebellious marriages to infidels, are others. But Paul does not cover such cases in 1 Corinthians 7. In that chapter, Paul was talking about new converts that were already married to unbelievers before becoming Christians, according to 1 Corinthians 7:17-24.
Idolatrous fornication
A Christian marriage to an unbeliever is idolatrous, spiritual fornication as demonstrated and identified in 2 Chronicles 21:11, "Moreover he made high places in the mountains of Judah, and caused the inhabitants of Jerusalem to commit fornication, and compelled Judah thereto." The high places are alters to other gods and by causing the inhabitants to worship at these high places, the Word of God identifies that this idolatry is fornication. By joining the body as the temple of God, to an idolatrous unbeliever (II Corinthians 6:16), one is clearly committing fornication in the sense identified in 2 Chronicles 21:11. Also because marriage represents Christ as the husband and the wife as the Church, would it be acceptable for the Church to be lead by another god or for Christ to be Head of an unbelieving assembly? This is spiritual fornication as mentioned in 2 Chronicles 21:11.
Vulnerable to divorce
Marriages to unbelievers are Biblically more vulnerable to divorce as demonstrated in 1 Corinthians 7. Referring back to the case of Deuteronomy 21:10-14, it seems unjust that the man has the discretion to divorce without reason just because he, “find no pleasure in her.” But in context of the whole Bible, this case actually has a reason for divorce and that is spiritual fornication, she is an unbeliever. However Jesus never said you must divorce for fornication, His word indicates that it was allowed but it was not required. Even Deuteronomy 24:1-4 uses the word “let” meaning allow, it does not command for divorce. When Joseph prepared to divorce Mary, it appeared Joseph chose to do so because under the circumstances he judged it better that Mary return to the father of her first child. Deuteronomy 21:10-14, does not require divorce but allows the elect discretion to choose divorce through the words, “find no pleasure in her.” That is discretion to execute divorce by reason of spiritual fornication. Even God symbolically referred to Himself as divorcing Israel for their fornications with other gods.
Jeremiah 3:8
And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce; yet her treacherous sister Judah feared not, but went and played the harlot also.
Isaiah 50:1
Thus saith the LORD, Where [is] the bill of your mother's divorcement, whom I have put away? or which of my creditors [is it] to whom I have sold you? Behold, for your iniquities have ye sold yourselves, and for your transgressions is your mother put away.
Ezra also is another example of divorce for breaking God's command against marrying unbelievers. Matthew 5:32 and Isaiah 50:1, clearly identify that divorce and “putting away” are synonymous; to interpret otherwise leads to absurdities. In that case Ezra the priest, organized for large numbers of divorces involving heathen. Some people argue by assumption that all these divorces regarded second marriages. There is no evidence to support that assumption. Ezra does not identify those men as divorcing their first wives to marry heathen wives, which should obviously have been condemned together with the sin of marrying unbelievers, if the assumption were true. It would have been absolutely ridiculous with all the condemnation talk about their sin of marrying heathen, not to include at least a mention or preferably condemnation of their greater sin of invalidly divorcing their Israeli wives to marry heathen women. Ezra 10:19 says, “And they gave their hands that they would put away their wives; and [being] guilty, [they offered] a ram of the flock for their trespass.” It does not say that they put away only their heathen wives but their wives [in general]. Because Ezra did not condemn them for divorcing Israeli wives, if any of them really did have Jewess wives, they must have engaged in legal polygamy, and thus there was no condemnation for putting away Hebrew wives because they would have kept them under polygamy. But marrying heathen spouses like Samson and Solomon did, was not legal. So because there were no invalid divorces in the Ezra case, but could have been polygamous marriages, it cannot be assumed that the men were all originally married to a Hebrew wife and that this is the reason why God allowed them to put away the second wives. Clearly the divorces were authorized because they had committed fornication by marrying heathen, which is defined by the Word of God in 2 Chronicles 21:11 as idolatry. Fornication is obviously an umbrella term mentioned by Jesus, as an exception clause which allows for divorce. There are other types of fornication such as this very disturbing one in 1 Corinthians 5:1. The Ezra divorce case does not contradict 1 Corinthians 7:12-15, because those verses are about unbelievers who married unbelievers before one of them became a follower of God, according to 1 Corinthians 7:17-24. With Ezra, it is a case of spiritual fornication, where followers of God had married unbelievers. So the Ezra divorces were judged as necessary because so many people had married heathen to such an extent that it was harmful for the spirituality of the society and their relationship with God. It appears that 1 Corinthians 7:12-15, is not a new law but might be a new clarification from God, which had previously been unnecessary in Old Testament times, because it was incredibly rare for gentiles to convert to Judaism. With Ruth she joined without an unbelieving spouse. I am not aware of any Old Testament precedent for a gentile converting to Judaism with a heathen spouse that refused to follow God. Ruth's deceased husband had committed sin by marrying a heathen, but this does not mean it was okay because there is no validity given to his behavior in the scriptures. He could have been chastised for backsliding, by early death. Also his brother married a heathen and suffered the same fate. His mother Naomi even admitted the Lord had not dealt well with her. So interfaith marriages to unbelievers are more vulnerable to Biblical divorce, compared with marriages between Christians.
Polygamy can be outlawed
Polygamy is valid if it is not illegal by the Law of the Land. The perfect will of God is for monogamy because in Genesis 2:24, it uses the singular words “man” and “wife”. Jesus pointed out that monogamy had always been the law since the “beginning”. But the permissive will of God allows for and recognizes polygamy in so many parts of the Bible. David, Abraham and Jacob engaged in polygamy, but God did not complain about their relationships, except when David committed adultery. David was manipulative and thought he could make his adulterous relationship with Bathsheba into a legal polygamous marriage, that God would not require him to break. This is comparable to manipulative Christians who deliberately rebel against God's commandments against marrying unbelievers, thinking that after they have committed their sin, they will not be required to divorce. In 2 Samuel 12:8, God specifically says that it was He who gave King David multiple wives and houses. God said if that was not enough He would have given more of “such and such things”. Just because God recognizes and allows polygamy does not mean that it is what God wants. David could not have received those wives without God, but that does not mean God wanted him to be polygamous. God also recognizes and allows divorce, yet God said He hates divorce. The perfect will of God is for monogamy. But those who sin by entering polygamous marriages must repent, yet God will still recognize those polygamous marriages, as valid and binding, if they have not been banned by the government of the country that has jurisdiction.
God as a “rubber stamp” for all marriages
It has been argued that Matthew 19:6, might prevent the validity of some mixed marriages except in cases like Esther and Xerxes, where God had clearly allowed for the marriage. The reasoning is that God is not a “rubber stamp” for marriage under Matthew 19:6, joining together all human initiated marriages to unbelievers. It is possible that because a human father in Numbers 30, has the authority to invalidate a daughter's vows and thus also demonstrates that the Word of God trumps human covenants, then perhaps also our Heavenly Father can invalidate vows like with King David, who swore to his own death but was confirmed by Nathan the prophet who said, “You will surely not die.” If God is not involved in Matthew 19:6, then that marriage becomes “what man has joined together” and might not be valid. We can speculate more on Matthew 19:6, but it makes no difference over the reality that God does not like Christians to marry unbelievers.
Losing your Christian soulmate
If you are too immature, perhaps even a false convert, and you marry an unbeliever, when God had someone especially planned for you like Rebekah and Isaac or Adam and Eve, who would have made you happier, your soul-mate will be given to someone else. If your soulmate was obedient, they will not be jeopardized by your foolishness, God planned everything in advance, so though they are your soulmate, their soulmate is someone else. So by choosing to marry an unbeliever, you are mostly harming yourself.
Conclusion
When an unbeliever marries an unbeliever, this is not a sinful interfaith marriage involving a Christian, even if one of them becomes a Christian afterwards. But if a Christian chooses to rebel against God by marrying an unbeliever, I honestly cannot advise what to do, other than for you to ask God.