The unforgivable sin refers to actions accompanied by an attitude that keeps a sinner from ever receiving God’s forgiveness. How could such a disposition develop?

 God forgives those who repent of their sins, follow his standards in their life, and exercise faith in Jesus Christ. (Acts 3:​19, 20) However, a person can become so set in following a sinful course that he will never change his attitude or conduct. The Bible describes such a person as having “a wicked heart” that has “become hardened by the deceptive power of sin.” (Hebrews 3:​12, 13) Like clay that has been fired in a kiln and that can never be reshaped, the person’s heart has become permanently opposed to God. (Isaiah 45:9) There could never be a basis for such a person to be forgiven, so he is guilty of the unforgivable, or unpardonable, sin.​—Hebrews 10:26, 27.

 Some Jewish religious leaders in Jesus’ day committed the unforgivable sin. They knew that God’s holy spirit was the force behind Jesus’ miracles, yet they maliciously said that he got his power from Satan the Devil.​—Mark 3:​22, 28-​30.

Examples of sins that can be forgiven

•  Blasphemy due to ignorance. The apostle Paul had once been a blasphemer, but he later said: “I was shown mercy because I acted in ignorance and with a lack of faith.”​—1 Timothy 1:​13.

•  Adultery. The Bible mentions some who at one time committed adultery but who changed their conduct and were forgiven by God.​—1 Corinthians 6:​9-​11.

“Have I committed the unforgivable sin?”

 If you sincerely hate your past course of sin and truly want to change, then you have not committed the unforgivable sin. God can forgive even repeated relapses into the same sin as long as your heart has not permanently hardened against him.​—Isaiah 1:18.

 Some people feel that they may have committed an unforgivable sin because they are troubled by nagging feelings of guilt. However, the Bible teaches that we cannot always trust our feelings. (Jeremiah 17:9) God has not authorized us to judge anyone​—even ourselves. (Romans 14:​4, 12) He can forgive us even when our own heart still condemns us.​—1 John 3:​19, 20.

Did Judas Iscariot commit an unforgivable sin?

 Yes, he did. His greed moved him to steal money that had been donated for a sacred use. He even pretended to be concerned about the poor when his real interest was to acquire more money to steal. (John 12:​4-8) Once Judas’ heart had become permanently set in doing wrong, he betrayed Jesus for 30 pieces of silver. Jesus knew that Judas could never truly repent for what he had done, and he called him “the son of destruction.” (John 17:12) This meant that when Judas died, he would suffer permanent destruction, with no hope for a resurrection.​—Mark 14:21.

 Judas did not show true repentance for his sin. He confessed, not to God, but to the religious leaders he had conspired with.​—Matthew 27:​3-5; 2 Corinthians 7:​10.