I believe what Jesus taught in John 5:24, that believers possess eternal life, which means from the MOMENT they believe they have it.
Then, Jesus taught in John 10:28 that recipients of eternal life (from the MOMENT of saving faith per 5:24) they SHALL NEVER PERISH.
This isn't difficult. When Jesus gives someone the free gift of eternal life, they shall never perish. And the gift is given to those who believe in Him, WHEN they believe.
So, whatever happens AFTER they have believed and received the free gift of eternal life, DOES NOT HAVE ANY AFFECT on their salvation.
However, God does not tolerate sin. Consider:
Heb 10-
26 If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left,
27 but only a fearful expectation of judgment and of raging fire that will consume the enemies of God.
28 Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses.
29 How much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace?
30 For we know him who said, “It is mine to avenge; I will repay,”
[d] and again, “The Lord will judge his people.”
[e]
31 It is a dreadful thing to fall into the hands of the living God.
This is about God's very painful discipline for His children who are unfaithful/disobedient. No one gets away with anything, which is the usual complaint of those who beieve OSNAS.