In 1 Corinthians 6:19-20, Paul wrote of individual Christians as being temples of God. In our passage, he refers to the church as a whole being the temple. Temples are holy places and should be protected against things that might defile the holy place, therefore we should protect our hearts and minds as holy places before the Lord. And the most logical thing to do to protect our hearts and minds is to not be unequally yoked with unbelievers.