I firmly believe God keeps His promise to anyone who is saved. But falling away can be more than just how you define it. It can be more than just being booted from a church and what you listed. It could be flat out rejecting God to have Him Lord of their life. If you tell God I no longer want you Lord of your life then God will do as you ask. I read nowhere that God says I am making you still be with me. Can you show me that verse that you seem to be applying here?
Well first thing-- at salvation.. we accept Jesus sacrifice on our behalf. It is nothing that we do to get salvation. He doesn't become Lord over every area of our life right then and there. It isn't a commitment to do good works. It's receiving His salvation because we CAN'T earn it. So.. this salvation doesn't belong to us. He owns it... He keeps it.
(Joh 10:28) And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
(Joh 10:29) My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand.
(2Ti 2:11) It is a faithful saying: For if we be dead with him, we shall also live with him:
(2Ti 2:12) If we suffer, we shall also reign with him: if we deny him, he also will deny us:
(2Ti 2:13) If we believe not, yet he abideth faithful: he cannot deny himself.
Is this Timothy verses contradicting ? No. The denial is the holding back of rewards by context. Plus it says if we believe not.. He abideth faithful.
The main thing being here is the indwelling of the Holy Spirit.
If I swallow an apple.. I can't then 'leave' the apple. It's actually inside me. The same thing goes with the Holy Spirit indwelling.. it's a physical possession.. the Holy Spirit is not actually outside of us.
(1Pe 1:3) Blessed be the God and Father of our Lord Jesus Christ, which according to his
abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
(1Pe 1:4) To an
inheritance incorruptible, and undefiled, and that fadeth not away,
reserved in heaven for you,
(1Pe 1:5) Who are
kept by the power of God through faith unto salvation ready to be revealed in the last time.
The last thing:
If someone goes off completely the opposite to Jesus, then it is fair to question what they believed to begin with. This isn't a 'license to sin'. As I said.. the Holy Spirit will discipline His children.
But He isn't going to LEAVE His child. Also.. 'God won't leave me, but I can leave God'.. is illogical.. because God would be staying with you in the process of you trying to leave Him! He KNOWS BETTER.