The thief who was on the cross next to Jesus and who believed Him and repented was promised a place in paradise. No unrepentant sinner can enter the kingdom of God. So, the "thief on the cross" was not exempt from the requirements of the Gospel. He became an ex-thief right where he was on the cross. His heart was made right with God. He feared God and understood righteousness (what is right according to God's will). He could speak like this to the other malefactor who was mocking Jesus: "Do you not even fear God, seeing you are under the same condemnation (as Jesus)? And we indeed justly, for we receive the due reward for our deeds; but this man has done nothing wrong” (Luke 23:40-41). By what came out of his mouth, it can be seen what kind of tree he was already.
Either make the tree good, and its fruit good, or make the tree bad, and its fruit bad; for the tree is known by its fruit.
43 For there is no good tree that brings forth rotten fruit; nor again a rotten tree that brings forth good fruit. 44 For each tree is known by its own fruit. For people do not gather figs from thorns, nor do they gather grapes from a bramble bush. 45 The good man out of the good treasure of his heart brings out that which is good, and the evil man out of the evil treasure brings out that which is evil, for out of the abundance of the heart, his mouth speaks.
The thief on the cross was justified under the same conditions as that man Jesus said was justified (next to the hypocrite Pharisee):
9 Also He spoke this parable to some who trusted in themselves that they were righteous, and despised others: 10 “Two men went up to the temple to pray, one a Pharisee and the other a tax collector. 11 The Pharisee stood and prayed thus with himself, ‘God, I thank You that I am not like other men—extortioners, unjust, adulterers, or even as this tax collector. 12 I fast twice a week; I give tithes of all that I possess.’ 13 And the tax collector, standing afar off, would not so much as raise his eyes to heaven, but beat his breast, saying, ‘God, be merciful to me a sinner!’ 14 I tell you, this man went down to his house justified rather than the other; for everyone who exalts himself will be humbled, and he who humbles himself will be exalted.”
The thief on the cross met the requirement of humility in repentance as much as this tax collector in Luke 18. Nobody is exempt from this rule:
“God resists the proud, but gives grace to the humble.” (1 Peter 5:5)
It’s the grace of God that brings salvation:
23 for all have sinned and fall short of the glory of God, 24 being justified freely by His grace through the redemption that is in Christ Jesus, 25 whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed (not present and future sins!), 26 to demonstrate at the present time His righteousness, that He might be just and the justifier of the one who has faith in Jesus.
Sin is in the past for the saved. Romans 3:24 comes after Romans 3:23. Anyone Romans 3:23 speaks to and who gets saved must identify with Romans 3:24, having been justified and redeemed from sin to walk in righteousness. The thief on the cross, if he had come down from the cross would not have in his heart to go on stealing. Jesus saw his heart. He could not mock God and pretend his heart was changed and then make his way into Paradise. The tax collector saying “have mercy on me, a sinner” in Luke 18 did not “go down to his house justified”, only to go on being the same sinner he had come to God as, that is, being a crook and mocking God, and planning already to come back and ask for forgiveness again and again for his future sins that people may suppose he had no choice but to continue doing (ex: bribing people, extorting money from people, stealing). No, he repented and went home with his sins forgiven and with a new way of life ahead of him, empowered by God’s grace.
8 For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, 9 not of works*, lest anyone should boast.
*Not of works of the Law but fruit worthy of repentance certainly (Matthew 3:8, Acts 26:20).
11 For the grace of God that brings salvation has appeared to all men, 12 teaching us that, saying ‘No’ to ungodliness and worldly lusts, we should live soberly, righteously, and godly in the present age, 13 looking for the blessed hope and glorious appearing of our great God and Savior Jesus Christ, 14 who gave Himself for us, that He might redeem us from every lawless deed and purify for Himself His own special people, zealous for good works.