There are various verses explaining that salvation is conditional on faith, and that to abandon faith means to fall into apostasy.
It's quite simple. A believer can never lose their salvation. And so the objective is remain a believer.
Romans 11:20-22
20 Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear. 21 For if God did not spare the natural branches, He may not spare you either. 22 Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off.
1 Corinthians 15:2 by which also you are saved, if you hold fast that word which I preached to you——unless you believed in vain.
Hebrews 3:14 For we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end
2 Timothy 2:12 If we endure, We shall also reign with Him. If we deny Him, He also will deny us.
Hebrews 6:4-6
It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Hebrews 10:29 How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace?
2 Peter 2:20-22 If they have escaped the corruption of the world by knowing our Lord and Saviour Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning. It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them.
Of them the proverbs are true: "A dog returns to its vomit," and, "A sow that is washed goes back to her wallowing in the mud."
Galatians 4:11 I am afraid for you, lest I have labored for you in vain.
1 Thessalonians 3:5 For this reason, when I could no longer endure it, I sent to know your faith, lest by some means the tempter had tempted you, and our labor might be in vain.
Hebrews 3:6 but Christ as a Son over His own house, whose house we are if we hold fast the confidence and the rejoicing of the hope firm to the end.
Colossians 1:21-23 And you, who once were alienated and enemies in your mind by wicked works, yet now He has reconciled in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister.
