The apostle John makes it clear that there are some in the church who claim faith in Christ who are not really believers in Christ.  And when they leave the church they also leave the faith — they are apostate (1 Jn. 2:18-19).

Thomas WatsonHow does that happen?  How do people become apostate?  Why do people give up on Christ?  Thomas Watson asks and answers the question this way:

Why do men thus run retrograde in their motion, and quite throw off that violence which they seemed once to have?

(1)  They never had a principle of spiritual life.
(2)  They never did the duties of religion with delight.
(3)  Through unbelief.
(4)  They love something else more than religion.
(5)  Out of cowardliness.
(6)  For lack of patience.
(7)  Out of the just judgment of God leaving them to themselves.

And of course Jesus also has a comment on those who believe and those who do not believe (Mt. 13:1-24).