For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Salem Markus Purba
All of the words of God will accomplish what God desires and achieve the purpose for wich God sent it (Isaiah 55: 10-11)
So is the word of God in John 3: 18 : "Whoever believes in him is not condemned, but whoever does not believe stand condemned already because he has not believed in the name of God's one and only Son".
James Kraft
The one thief repented. He changed his mind. He believed Jesus was the Son of God. The other thief did not.
John 6:47. Verily, verily, I say unto you, he that BELIEVETH on me HATH EVERLASTING LIFE.
First Corinthians 1:18 and 1:21. The preaching of the cross is to them that perish foolishness, but unto us it is the POWER of God.
For after that, the world by wisdom knew not God, It pleased God by the foolishness of preaching to save them that BELIEVE.
The GIFT of God is eternal life to all who just believe on His name. John 1:12. Romans 1:16. For I am not ashamed of the gospel of Christ, for it is the POWER of God unto salvation to EVRYONE THAT BELIEVETH, To the Jew first, and also to the Greek.
The thief on the cross repented, metanoia, and accepted Jesus as the savior of the world. He went from unbelief to belief. That is repenting. It is a change of mind. We quit trusting in what we do to save us and trust totally in what Jesus did to save us.
Acts 16:31. John 6:47. First John 5:13. These things have I written unto you that BELIEVE on the name of the Son of God, that ye may KNOW YE HAVE ETERNAL LIFE.
We know we have ETERNAL Life for several reasons. God said so, and it is a free GIFT from God by a one time FAITH alone in Jesus alone, apart from any works.
Roman 6:23 For the wages of sin is death, physical death, but the GIFT of God is eternal life, through Jesus Christ our Lord.
Since it is a GIFT, it is paid in full by the GIVER. John 10:28-29
Romans 3:26-28 You can not boast in a free gift that we did nothing for or deserve. John 6:40 And this is the will of Him who sent me, that ALL that seeth the Son, and believeth on Him, may have everlasting life and I will raise him up on the last day.
Gods word, and He can not lie. First John 2:2. For He is the propitiation for our sins and not for ours only, but for the sins of the whole world. But, only those who accept the free gift are saved. John 3:18.
The thief did nothing but believe on Jesus as his only hope of heaven.
He knew he deserved to go to hell. He was a thief. He may have also been a murderer. The thing is he was a sinner just like us. Jesus paid the debt of sin he owed God and gave him eternal life.
The just for the unjust. Romans 3:26-28. For He is the just and the justifier of them that BELIEVE in Jesus. Where is boasting then? It is excluded. By what law? By the law of works? Nay, But by the law of FAITH. Therefore we conclude that a man is justified by FAITH apart from the deeds of the Law.
Jesus paid the debt and set us free from the old law of sin and death. Stamped. PAID IN FULL. John 3:18. No longer condemned.