Some Christians sing the right song while sitting in pews, but their heart is far from God. Their heart is cold. Once on fire for God, they've lost their appetite for Godly things. Serving God has become a burdon. Soon they will be drawn to worldly things.

And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good and acceptable and perfect will of God.   Romans 12:2

Cold hearts cannot discern the spirit of God until they renew their mind. If we don't renew our minds, we will get farther away from God and His will for our lives. 

We must renew our heart daily through Bible study, spiritual songs, and prayer.