Biblically, if you renounce (give up, relinquish, abandon, walk away from) your salvation, you were not saved to begin with. Biblical sorrow, which is humility and contriteness of heart and spirit leads to a repentance not to be repented of. True belief, faith that is living will never allow you to "un-believe." If you preached from a pulpit for 50 years, then wake up one morning and say, "God is not real, none of this is true, and I am not a Christian," you are not de-converting, you are simply revealing what you've always been: a fraud. The Apostle John said, "...they were not of us," not, "...they used to be of us." There is no such thing as a used-to-be Christian, only a non-Christian. You're one or the other and nothing in between. This is one of those dangers that comes with easy prayerism, easy believism, and the 1-2-3-pray-after-me evangelism. Salvation is a heart issue. It is a conversion from old to new. We can mimic conversion to an extent and for only so long; but true conversion is God's handiwork. It is evident, and it is eternal.