Jesus’ teaching in Matthew 19:3-9 raised eyebrows. His disciples responded that it seems better not to marry if this is how marriage works. Jesus did not deny the truth of their statement and described that some would live a celibate life to be pleasing in the sight of God (Matthew 19:11-12).
When God created the first man and woman, He brought them together in marriage (Genesis 2:22-24). God has identified the details of the marriage covenant, including the restrictions and freedoms it brings to the two made one. Ungodly people have often become married but have not maintained God’s expectations for their covenant. Many marry, divorce, and marry again for reasons God never permitted in the Scriptures (Matthew 5:32; 19:9). God says those who do this are living in adultery. They must repent to be forgiven by Him.
I have heard it taught that non-Christians living in adulterous marriages need only be baptized to “cleanse” their marriage. Here are two scriptural objections to that teaching:
First, marriage is not a “Christian thing.” Before the Old and New Testaments, God designed marriage and gave it to us (Matthew 19:4, 8). When two eligible people become married, God makes them one (Matthew 19:6). Becoming a Christian does not alter or remove your marriage covenant.
Second, baptism does not make adultery, nor any other sin, acceptable in the sight of God. Acts 2:38 commands that repentance be paired with baptism in the name of Jesus to receive forgiveness of your sins. Adultery is a sin, and the only way to repent of it is to stop committing adultery.
If your marriage is identified as adulterous according to Matthew 5:32 or Matthew 19:9, you must repent of your adultery (stop committing adultery and end the relationship). Then you can be forgiven and justified by the blood of Christ in baptism.