Those who are still trusting in what they consider their own good works for salvation are not trusting in Christ.
When Christ changes us into a new creation it is He that does it not ourselves. He changes our nature entirely, the old nature is gone. We can never go back to the old nature again.
Hebrews 12:5-7 "…5 And you have forgotten the exhortation that addresses you as sons: “My son, do not take lightly the discipline of the Lord, and do not lose heart when He rebukes you. 6 For the Lord disciplines the one He loves, and He chastises every son He receives.” 7 Endure suffering as discipline; God is treating you as sons. For what son is not disciplined by his father?"
God promised that we are eternally saved, that means forever when we become born again.
There are no scriptures that say the Holy Spirit Who resides in the born again believer ever leaves us.
"Make sure that your character is free from the love of money, being content with what you have; for He Himself has said, “I will never desert you, nor will I ever abandon you,”
He told us that we are His for eternity.
Ephesians 1:13-14
"13 In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of the promise,
14 who is a first installment of our inheritance, in regard to the redemption of God’s own possession, to the praise of His glory."
The scriptures that say someone walks away from God are clearly those who have never been changed by Christ, they decided on their own without trusting in Him to join the "Christian" group wanting to come in their own way. These are those who are described in Matthew 7:21-23 where God says; "I NEVER knew you."
Matthew 7:21-23 "21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’"
Most people in our day who would say they are Christians are pretenders who have never been born again. Those who walk away do it because they do not have the strength of the Holy Spirit to follow Him, they are walking in their own fleshly way without Christ.
Christ paid it all on the cross for us, we have no part in our own salvation except to agree with God that we are sinful and desire to be changed by Him.