Think about it like this…
(1) A unbeliever sins because they are not born again and sin is the fruit of their unbelief.
Romans 14:23, “…everything that does not come from faith is sin.”
(2) A believer lives holy because they are born again and righteousness is the fruit of their faith.
Colossians 1:5-6, “5 the faith and love that spring from the hope stored up for you in heaven and about which you have already heard in the true message of the gospel 6 that has come to you. In the same way, the gospel is bearing fruit and growing throughout the whole world—just as it has been doing among you since the day you heard it and truly understood God’s grace.”
(3) However, when a sinner stops disbelieving and starts having faith in Jesus they are transformed from sinner to saint.
Ephesians 1:13-14, “13 And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit, 14 who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.”
(4) And at the same time, when a saint stops believing and starts sinning without repentance they will be hardened in their heart because of unbelief and then cut off and treated as a sinner.
(a) Hebrews 3:12-14, “12 See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God. 13 But encourage one another daily, as long as it is called “Today,” so that none of you may be hardened by sin’s deceitfulness. 14 We have come to share in Christ, if indeed we hold our original conviction firmly to the very end.”
(b) 2 Peter 2:21-22, “21 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. 22 Of them the proverbs are true: “A dog returns to its vomit,” and, “A sow that is washed returns to her wallowing in the mud.”
(c) Hebrews 10:26-29, “26 If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, 27 but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. 28 Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. 29 How much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace?”
(d) John 15:5-6, “5 I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing. 6 If you do not remain in me, you are like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned.”
AN EXAMPLE FROM ROMANS 11:11-24
The Jews in Romans 11 went from being sinners (by birth) to saints (in covenant) back to sinners (because of sin and unbelief- breaking the covenant/law), and Paul ended by saying if they go back to faith- but now in Jesus (New Covenant) they will be grafted in again. And the Gentiles who were sinners (by birth), now were saints (because of faith in Jesus), they were warned to continue in faith, lest they return to unbelief and be cut off as sinners.
Check the text:
Romans 11:20-23, “20 Granted. But they [the Jews] were broken off because of unbelief [that means they were once in], and you [Gentiles] stand by faith. Do not be arrogant, but tremble. 21 For if God did not spare the natural branches [Jews], he will not spare you either. 22 Consider therefore the kindness and sternness of God: sternness to those who fell [unbelievers/sinners], but kindness to you [believers/saints], provided that you continue in his kindness. Otherwise, you also will be cut off. 23 And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.”
Sinners become saints by belief in Jesus (with the fruit of righteousness) and saints become sinners by unbelief in Jesus (with the fruit of sin).