Make no mistake, God is Sovereign.
Psalm 115:3
But our God is in the heavens; He does whatever He pleases.
Those who give up on their faith, turn apostate, if they were Christian in the first place, may have given up on God. But Jesus never gives up on them. Those are still in the palm of his hand as he promised. Because eternal salvation is exactly what the Christ who died to deliver the seal on that promise said it is. Eternal.
John 10:28
That isn't speaking of temporary life. Throw away salvation at the whim of the saved. It is speaking as the son of God, indwelt by the spirit of the most holy of fathers, giving a guarantee sanctified by blood on the cross, that it is what Jesus died to insure it would be for all time.
While those who claim to be Christian and really are not in the covenant, those are not saved in the first place. Their kind was warned of when they appeared amid the disciples and then walked away.
1 John 2:19
The saved in Christ are reborn, they're not un-done in all that salvation merits if they stray or turn from the path of righteousness. Because God called them to his son. And they arrived. No one can take us from Jesus' hand, we can't even leap from it of our own accord when we are not there by our own work in the first place. We're there by God's grace.
He wrote our name in the book of life before the world came to be. He knew us before the womb. And he may have inspired the invention of White-Out, but he doesn't use it in the book of life. We are eternally saved by the power of the holy spirit. If we could change that, our power would be greater than his. And that just is not true.