True Christians DO NOT Depart
First let us define apostasy. Apostasy means a total desertion of or departure from one's religion, principles, party, cause, etc. which is different from the backslider. So it would be terminological error to say, "David didn't backslide, he just sinned. He never stopped believing in God." because a backslider is somebody who is saved but he/she is inconsistent with fruit bearing and obedience, not so with the apostate!
One can ask, "Can a Christian trust Jesus today and then... something or someone else tomorrow?". The answer is no. Huh? Why? How? First and foremost, salvation does not only save the person, it CHANGES the life of the person and enduring to the end is only normal for a saved person even if the person today may not be living appropriately still, that person will endure to the end- which will fit Matthew 24:13 correctly as to show who's been saved vs. who's been the pretender. 1 John 2:19 says, "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us."
So what does that mean? There is NO ex-believer. True believers will continue to believe on the Lord Jesus Christ as a result of them being saved. They will NEVER trust their works for salvation, their works come out ONLY because Jesus Christ saved them and it's by grace (Titus 2:11-15) and there is NO boasting even (Romans 3:29) if they do a million good works during their lifetime.
It would be good to use Judas Iscariot as an example. He was NEVER a Christian to begin with. John 17 mentions the facts about the disciples in verse 12 saying, "While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the Scripture might be fulfilled." which can only refer to Judas Iscariot. His actions showed he was not saved to start with:
Note: Judas Iscariot being called the "son of perdition" does not make him the Antichrist. He was the son of perdition in the days of Jesus' earthly ministry. He is a type of Antichrist just as Caiaphas is!
- He was an unrepentant thief (John 12:15-16)- Christians have repented of their sins, if they ever stole anything, they would repent and return, restore but no not Judas Iscariot. It was already his lifestyle!
- He sold Jesus for 30 pieces of silver- no Christian would EVER DO THAT- Peter denied but later repented to God. Judas Iscariot's "repentance" was NOT genuine.
- Suicide proved he was not saved. Most people who kill themselves are confirmed to be NOT saved in the first place. Sad but true.
So therefore, if somebody says, "I used to be a Christian... then I became a (insert false religion here)." they are LYING! Christianity is NOT a manmade religion like Catholicism or Mormonism- it is believing on the Lord Jesus Christ for salvation which results to discipleship as the disciples were called as Christians in Antioch (Acts 11:23-26). Christians inevitably deny themselves further because during the saving process, they deny themselves to the Lord Jesus Christ putting aside their filthy rags and trusting in Him ALONE for salvation. They repent (change their mind) about their sins and who Jesus is- no repentance of sin, no repentance of unbelief. The apostates of today expose themselves (like how certain "Baptists" became Roman Catholics)- they reveal they were NEVER Christians to start with!