Be ye not unequally yoked together with unbelievers: for what fellowship (in common) hath righteousness with unrighteousness? and what communion hath light with darkness – 2nd Corinthians 6:14.
The caution itself, not to mingle with unbelievers, not to be unequally yoked with them was inspired by God to the Apostle Paul. It is wrong for good people to join in affinity with the wicked and profane. There is more danger that the bad will damage the good than hope that the goodwill benefit the bad. We should not yoke ourselves in friendship with the wicked men and unbelievers. We should never choose them for our bosom-friends. Much less should we join in religious communion with them?
Believers are righteous when they submit themselves to the instructions of the Bible, but unbelievers are unrighteous according to the teaching of the Bible. What comfortable communion can these have together? Jesus Christ and Belial are contrary one to another. We must take care not to defile ourselves by converse with those who defile themselves with sin. This is why the Bible has instructed the believer not to associate themselves by being running buddies with those who reject the word of God (Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing, and I will receive you – 2nd Corinthians 6:17).
Paul wasn’t encouraging isolation from unbelievers, but discouraging compromise with their sinful values and practices. He was urging them to maintain integrity in the world just as Jesus did. Paul was exhorting the Corinthians to cleanse themselves from all filthiness. In other words, they were to wash away all dirt from both flesh and spirit, meaning those actions and attitudes that came from having the false teaching in their midst. The point of cleansing is perfecting holiness. Meaning, dedicating ourselves to Christ and living righteously.
Unequally yoke is also marrying someone you know is an unbeliever of the faith. This is one of the reasons why God instructed the children of Israel not to marry outside of their nation. Intermarriage in ancient Israel was not racial nor ethnic but spiritual: lack of faithfulness to God Himself. Marriage is spiritual, it’s an example of our relationship with the Father and Son and our faithfulness to them. God doesn’t want us to serve any other god other than Him; neither should we worship any other god other than Him.
When marrying an unbeliever, we’re submitting ourselves to one that serves the god of this world. Their beliefs are different from that of the believer. No matter how good the person may be, their hearts haven’t been changed by the Spirit of God, so their actions and beliefs will be opposite of the word of God. According to the teaching of Jesus, a house divided can’t stand (And if a house be divided against itself, that house cannot stand – Mark 3:25). Jesus’ parable was actually a threefold message that contrasted unity and disunity. Nothing including Satan’s kingdom can stand if it is divided.
There are two spirits, God’s Spirit and the spirit of Satan, we can’t serve both, we’re either serving the Spirit of God or the spirit of Satan. Marrying an unbeliever and being married to an unbeliever is different. Why some people get saved, and their spouse get saved soon after, no one knows, but God. Being married to an unbeliever can be a trying of your faith, it doesn’t mean you should divorce your spouse because they’re unbelievers. Pray for your spouse’s salvation, never badger them with the word of God, don’t nag them to attend church with you. Live a life that represents God, and God will do the rest!