The short answer is that the Bible tells Christians to marry in the faith, not outside of it. If a Christian chooses to marry, he or she is to marry someone who “belongs to the Lord” (1 Corinthians 7:39).
I realize some see this as rather restrictive. They question why God would limit a Christian’s options when it comes to finding a marriage partner. So let me explain the reason for the restriction.
God designed marriage to be characterized by oneness. He wanted a man and woman to experience oneness in spirit, soul and body. This emphasis on oneness comes out in Genesis 2:24: “For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.” Jesus emphasized oneness in marriage when he said a husband and wife are “no longer two, but one” (Matthew 19:6).
Now anyone who’s married will tell you that achieving oneness in marriage is not easy or automatic. We all enter marriage with a stubborn tendency to be self-centered. And our selfishness hinders oneness. If oneness is challenging when both husband and wife are Christians, it’s infinitely harder when one partner doesn’t belong to the Lord Jesus. The couple may have much in common, but there will be a disconnect on the spiritual level that puts true oneness out of reach. That’s why Christians are to marry in the faith.
What should Christians do who have already married outside the faith? The Bible is clear that God expects them to stay in the marriage and work at making it as solid and satisfying as possible (1 Corinthians 7:12-13). They should also seek to quietly live out their faith (1 Peter 3:1), trusting that their partner may eventually choose to join them in following Jesus.
(This article is from Dr. Reed’s book, That’s a Good Question)