I asked that question to someone recently, who answered, "because they want to".
Hebrews 3:14 KJV
14) For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
We look at verses like this, and many explain them as saying, you remain in Christ so long as you remain believing. Is that what the passage says? No, it doesn't. It is saying you've become a permanent partaker of Christ, and that this is shown by the fact that you hold your confidence to the end. It's there in the grammar.
Whether or not you think that someone could "lose their salvation",
Here's my question.
The Christian who remains believing, lifelong, why do they do that? Why would a believer NOT abandon their faith? Why is it that a Christian remains believing?
My answer is that as we've received Christ, we now have the faith of Christ, and He keeps us safe. That having been reborn in His pattern, we will always trust Him, even if only in the inner man.
What other answers could there be?
Does the one continue in faith because they are a better, stronger person? God has chosen the weak.
Does the one continue in faith because they have obtained a better salvation? We have obtained like precious faith.
What other answers can you think of?
1 Peter 1:3-5 KJV
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,
4) To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
5) Who are kept by the power of God through faith unto salvation ready to be revealed in the last time.
Much love!
I think I will go with what the apostle Paul said so many times in his letters. Let's see, I will go with this one today.
Col. 1:21-23
"And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled
22 In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight:
23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;"
So, we see one can be moved away from their faith, but what does the Apostle say about how this happens?
Col. 2:4-8
"And this I say, lest any man should beguile you with enticing words.
5 For though I be absent in the flesh, yet am I with you in the spirit, joying and beholding your order, and the stedfastness of your faith in Christ.
6 As ye have therefore received Christ Jesus the Lord, so walk ye in him:
7 Rooted and built up in him, and stablished in the faith, as ye have been taught, abounding therein with thanksgiving.
8 Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ."
So, here we see as in so many of the great Apostles letters, that one can be deceived by false teachers to depart from their faith in Christ.
If you deny this is possible, you've got some serious problems with our Lord!