A believer can become an unbeliever.

If an unbeliever can become a believer then why can’t a believer become an unbeliever?  He/she can and the Bible shows this.

“Some shall depart from the faith” (1 Tim. 4:1). A child of God can deny the faith and become “worse than an infidel” (1 Tim. 5:8).  Can one who is worse than an infidel enter heaven?  False teachers can “overthrow (destroy) the faith of some” (2 Tim. 2:18); and “some…have made shipwreck” of their faith (1 Tim. 1:19)

“Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God” (Heb. 3:12).  This warning was given to brethren (“holy brethren, partakers of the heavenly calling,” (Heb. 3:1), undeniably children of God), “lest there be in any of you….” Children of God – believers – can become unbelievers and depart from God.  Can one go to heaven with “an evil heart of unbelief” (cf. Heb. 11:6; Rev. 21:8)?