Base Your Relationships on God
Matthew 24:12 "And because iniquity shall abound, the love of many shall wax cold."
The only reason that an abundance of iniquity would make anyone's love for the Lord wax cold, is if he were not separated from that iniquity. As with Lot, a righteous man can vex his soul from day to day by hearing and seeing the unlawful deeds of the ungodly (2 Pet. 2:8).
We have to deal with the world's system and those in it, but we need to be careful and maintain proper balance. We should shun relationships with those who influence us negatively more than we influence them positively.
Marriage is one area where this principle is especially true. There is no closer union in life than the marriage relationship. A believer who marries an unbeliever is directly violating this scripture and is toying with disaster. The Lord should be the most important person in our life. How could we possibly become "one" with a person who doesn't love our Lord?
It is folly to think that after marriage an unbelieving spouse will accept the Lord. Although that does happen, it cannot be guaranteed. In fact, statistics are overwhelmingly against this happening. Regardless of that possibility, a believer who marries an unbeliever is directly violating God's instructions here. It is not a good start to any marriage to spurn God's Word.
Being unequally yoked with unbelievers just doesn't work. Believers and unbelievers are as different as righteousness and unrighteousness, light and dark, Christ and the devil. One has faith and the other has none. One is the temple of God and the other an idol. Any Christian who doesn't see this conflict is deceived.