God wants His people to enjoy harmony and agreement

. . . as we live and work together. When we try to walk with the world and with the Lord at the same time, we break this spiritual fellowship and create discord and division. 

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 a saved person marries an unsaved partner, it sets up an impossible situation and the same thing applies to business partnerships and religious fellowships.

God commanded His people to "Come out" -- "Be ye separate". Separation is not just a negative act of departure, its also a positive act of dedication to God.

In our desire for doctrinal and personal purity, we must not become so self-centered that we ignore the needy world around us. Our Lord was holy, harmless, undefiled, and separate from sinners, and yet He was a friend of publicans and sinners.

We must practice contact without contamination. Otherwise, we will isolate ourselves from the people who need our ministry the most.