Can a believer and an unbeliever live together in harmony?
No, a believer and an unbeliever cannot live in harmony. They are total opposites and cannot pull together. Living under the same roof would require much grace, yet they have no spiritual agreement, for one has Christ and the other is an infidel who cannot claim God's promises.
Les explains that Belial means worthless, and it is ridiculous to join Christ—who is the epitome of goodness, grace, righteousness, and majesty—with something worthless, just as ridiculous as putting a donkey beside an ox. He notes that a believer and an infidel are two total opposites and cannot pull together, so living under the same roof would take a lot of grace. Verse 16 asks what agreement the temple of God has with idols, and Les says the temple of God is the believer's physical body where God dwells. God promises to be their God and they His people, but the unbeliever cannot claim that promise.
And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?