When a person truly repents and places his trust in the Savior for forgiveness and everlasting life he is changed by God; born again. “Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new” (2 Corinthians 5:17). “Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life” (John 5:24). Keeping salvation is not dependent on the Christian. As it is written: “Now He who establishes us with you in Christ and has anointed us is God, who also has sealed us and given us the Spirit in our hearts as a guarantee” (2 Corinthians 1:21-22) and "He who has begun a good work in you will complete it until the day of Jesus Christ;” (Philippians 1:6; cf. Ephesians 1:13-14). If He began the work, He will complete it. That’s “if” He began the work. It’s quite prideful and arrogant to think you can be good enough or do something to keep that which you couldn’t earn in the first place. You didn’t deserve/merit it when you got it and you never will be able to.
Nonetheless, those who are born again will endure to the end. If they don’t, then that’s proof that they weren’t truly born again (1 John 2:19). By the way, if you teach that a person can lose his/her salvation, you’re essentially teaching that a person can/ is able to go on sinning. Granted they might eventually lose their salvation, but nonetheless they can go on sinning according to that false theology. However, the Bible clearly teaches that a true saint will not and can not go on living in habitual unrepentant sin (1 John 3:9; 5:18; Romans 6:1-2).
Some Questions to Consider
How many sins do you have to commit before God takes your salvation away (how many are covered by the blood of Christ)? We are told, “where sin abounded, grace abounded much more” (Romans 5:20; read in context of Romans 6:1-2). How many sins before God remembers your sin and lawless deeds again (Hebrews 10:17; Jeremiah 31:34)? What trespass separates us from God? We are told He has forgiven “all trespasses” (Colossians 2:13) when He made us alive with Christ. Didn’t Christ die for all our sins (past, present and future)? At what point does Jesus stop acting as our Advocate (1 John 2:1)? If God gives a person everlasting life, then it ends, was it everlasting (John 3:36)? If a person is born again of “imperishable” seed (1 Peter 1:23), then that person perishes, was the seed really imperishable? Does God unforgive a person’s sin (Psalm 103:12; Isaiah 43:25; Jeremiah 31:34; Hebrews 8:12)? Does God remove His wrath from a person then reapply it when that person loses their salvation; or does a saved person have the wrath of the wrath of God abiding on him? Doesn‘t God know who‘s going to stay justified? Isn’t God able to put His “fear in their hearts so that they will not depart” as Jeremiah 32:40 speaks of? In John 10:27-29 Jesus said, “My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand.” Who shall never perish? Jesus didn’t say “follow Me to become one of My Sheep.” He said His Sheep listen to and follow Him. He didn’t say they might follow Him, He said they do follow Him. Also, consider Isaiah 43:13 with John 10:27-29. One more question to consider, are you trying to live your life in a way so that you will go to heaven or are you simply depending/relying on God for eternal life?
Those whom God “foreknew, He also predestined” (Romans 8:29) and those “He predestined, these He also called” (Romans 8:30) and the ones “He called, these He also justified; and whom He justified, these He also glorified” (Romans 8:30). This is an indivisible chain of events. If God foreknew a person, that person will ultimately be glorified.
“Are you saying a person can get saved, then go rob banks, fornicate, murder and sin all they want and still be saved?”
Paul put it like this, “What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it?” (Romans 6:1-2). The true Christian won’t go on sinning because he has been born again. As it is written, “Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God” (1 John 3:9) and “We know that no one who is born of God sins; but He who was born of God keeps him, and the evil one does not touch him” (1 John 5:18). We know from the Greek that these refer to continuing in sin; making a practice of sin. Thus, if you continue in unrepentant sin, all evidence indicates that you didn’t get saved in the first place.
“Don’t Christians have free will; can’t they turn away from God (choose not to be saved)?”
Christian have a degree of free will, but they also have the fear of God so that they will not turn away. As it is written in Jeremiah 32:40, “And I will make an everlasting covenant with them, that I will not turn away from doing them good; but I will put My fear in their hearts so that they will not depart from Me.” Thus, if you turn away from God and the faith, then you weren’t saved to begin with. As it is written, “They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us” (1 John 2:19).