In Matthew 12 the scribes and Pharisees came to Jesus and tempted him. They asked him to give them a sign to prove that he is the Messiah. Jesus responded by saying the following:

An evil and adulterous generation seeks after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth. The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here. ~ Matthew 12:39-41

The reference to the sign of the prophet Jonas (Jonah) can be found in Matthew 12:38-41, Matthew 16:1-4 and in Luke 11:29-32 and is an expression employed by Jesus to describe his death and resurrection, which was “the sign” to that generation that he indeed is the Messiah.

The scriptures foretold that the Messiah would be rejected by his generation and resurrected after suffering at their hands.

In Matthew 12:38-41, Jesus says that Nineveh repented in response to the preaching by Jonah after Jonah had spent three days and three nights in the belly of the whale. According to Jesus, the people of Nineveh are going to rise up in the day of Judgment and condemn those who rejected Jesus during his generation because they would not repent.

For as Jonas was a sign unto the Ninevites, so shall also the Son of man be to this generation. ~ LUKE 11:30

According to Jesus, the only sign God was giving to the unbelieving Jews was his death and resurrection. If they refused to repent upon hearing the gospel preached in the power of the Holy Spirit, they would not be saved.

After his resurrection, Jesus enabled the apostles as well as other believers to give testimony to his resurrection through the power of the Holy Spirit. Throughout the book of Acts, those who preached the gospel suffered much persecution from the religious Jews who wanted to stop the work of the Holy Spirit from giving testimony and witness to Jesus’s resurrection.

The hatred for Jesus by the wicked leaders of the Jewish people and the religious Jews who refused to repent, is the context in Matthew 12 and the context of the unpardonable sin which is mentioned in verse 31. When Jesus speaks of the sin against the Holy Spirit he is referring to the sin committed by the religious Jewish leaders who rejected Christ as Messiah.

In his presence they called him Master (v.38) but behind his back they called him the instrument of Beelzebub because they wanted to destroy him (v.14). However, Jesus knew how truly wicked they were and called them  “a generation of vipers” (v.34).

They did not have it in them to speak well of him because their hearts were evil and unrepentant. They resisted the Holy Spirit and spoke against him by their speaking against his work through the Son of God.

When Jesus made the statement about the unpardonable sin it was to the hard hearted unrepentant religious Jews who refused to repent. They resisted the Holy Spirit in Christ and despised the Spirit of grace working through him; and they persecuted his followers (their Jewish brethren) in the Book of Acts.

It is within this context that Jesus says, there is no forgiveness for them. They rejected the one whom God sent and anointed. This is not a sin that can be committed in ignorance, or accidentally. It refers to the deliberate rejection from their hearts of Jesus whom the Holy Spirit had anointed. They so hated Jesus that they spoke evil against the work of the Holy Spirit in and through Jesus. They had no fear of God in them.

This happened just as Isaiah the prophet had predicted.

Go, and tell this people, hear ye indeed, but understand not; and see ye indeed, but perceive not. Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed  (Isaiah 6:9-10; Matthew 13:13-15;Mark 4:12; Luke 8:10; John 12:40; Acts 28:26-27; Romans 11:8; 2 Corinthians 3:14-15).

Jesus said to them: “If ye were blind, ye should have no sin: but now ye say, we see; therefore your sin remaineth”. ~ John 9:41

This is what happened to the leaders of Israel who rejected Jesus and put him to death. They closed their ears, they shut their eyes, and they hardened their own hearts. They refused to believe on the one whom God had sent.

Jesus warned them that the queen of the south would rise up in the judgment against them and condemn them because “she came from the uttermost parts of the earth to hear the Wisdom of Solomon; and He who is greater than Solomon was in their midst.” (v.42)