2 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*?
When God says we should not be unequally yoked with unbelievers, the picture that comes to mind is that of oxen whose yokes are not equal. They will struggle to walk together and both of them will be stressed because the yoke binding them is not equal.
That is what happens If a believer in Christ marries an unbeliever. In certain aspect of their marriage, there is a stress they experience because there is no equality in spiritual persuasion. Light and darkness are unequal and they are opposite.
There is something we call unity in spiritual things. This can be achieved when two or more people are of the same faith and spiritual persuasion. In marriage, this spiritual bonding is expected of all couples but it can only achieved when they are of the same faith and doctrinal persuasion.
There is power in spiritual unity. I know what happens when I tell my wife and children to agree with me in prayer. That wouldn't happen in a home where there is no spiritual unity.
Sometimes, certain issues require we join forces together and put the enemy to flight. There is a limit to how far one person can go but there is no limit to the mileage you can cover if forces are combined.
The bible says, *"Again, truly I tell you that if two of you on earth agree about anything they ask for, it will be done for them by my Father in heaven*" ( Mt 18:19)
It makes deep sense when the two in marriage call on God the father of our Lord Jesus Christ. That spiritual bonding will always have a positive effect on other aspect of their marriage.
This happens only when two believers marry. If believers and unbelievers marry, they cannot enjoy this aspect of marriage.