Let us explore the exegetical evidence for what is truly the unforgivable sin in the bible. According to those gospels, Jesus had actually just cured a blind and mute guy who was demon-possessed:
Then they brought to Him a demon-possessed man who was blind and mute. Jesus healed him so that he could speak and see. Mat 12:22
A man who is blind and mute can neither see, speak, nor hear. The response of those who saw this miracle is exceptionally crucial. To begin with, the response of the common people:
All the crowds were amazed and said, “Could this one be the Son of David?” Mat 12:23
Why would the people, after this specific miracle, ask this question? It was a Jewish custom that solely the Messiah will have the ability to cure a blind mute, due to the fact that a man that can not see, hear, or speak would not have the ability to answer in any way to the Messiah’s commands to be cured. For that reason, the Messiah would need to be speaking straight to the satanic force inside the man. This is why the crowds asked if Jesus was “the Son of David”, a Messianic term. In essence, the people understood that this man must be the Messiah by His power to perform this specific miracle. So, the Pharisees understood that there was only one other possibility. Note their reaction to the crowd’s question about Jesus being the Son of David, the Messiah:
But when the Pharisees heard this they said, “He does not cast out demons except by the power of Beelzebul, the ruler of demons!” Mat 12:24
This allegation exposes the desperate condition of the religious leaders to reject Jesus and hold on to their authority over the people. They understood full well, even much better than the common people, that the power to do this miracle might just originate from God through the Messiah. Unless they could possibly make the people in some way think that Jesus was in league with Satan himself, the Jewish leaders would have been required to acknowledge that He was the Messiah.
Jesus reacted to the claim that He carried out the miracle by being in league with Beelzebul with words that are a few of the most discussed and misconstrued words in the Bible. First off, He explained the absurdity of Satan casting out his own demons, destroying the power of his own kingdom. Second, He clarified the repercussions of the allegation that they had actually just leveled upon Him:
For this reason I tell you, people will be forgiven for every sin and blasphemy, but the blasphemy against the Spirit will not be forgiven. Mat 12:31
Whoever speaks a word against the Son of Man will be forgiven. But whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. Mat 12:32
With their backs to the wall, the Pharisees needed to save face in front of the people by rejecting Jesus was the Messiah in performing this miracle, associating His power to Satan himself. In doing so, they sealed their fate by designating to Satan what the Holy Spirit had actually just done. This is the unforgivable sin; the sin of blasphemy against the Holy Spirit, and it can not be forgiven. In danger of losing all trustworthiness with the people gathered around this scene, see the response of a few of the Pharisees and experts in the law:
Then some of the experts in the law along with some Pharisees answered Him, “Teacher, we want to see a sign from you.” Mat 12:38
Still defiant, they asked Jesus for a sign. This came just after Christ gave the greatest sign: casting the demon out of a man who could not see, speak, or hear.
What they wanted was a sign that the source of His power was not from Satan. A sign that He was definitely deity, such as calling down fire from heaven like Elijah, or some other supernatural, unmistakable sign from God. Keep in mind the answer Jesus provided:
But He answered them, “An evil and adulterous generation asks for a sign, but no sign will be given to it except the sign of the prophet Jonah. Mat 12:39
For just as Jonah was in the belly of the huge fish for three days and three nights, so the Son of Man will be in the heart of the earth for three days and three nights. Mat 12:40
There was going to be no fireball from heaven that day for the Pharisees and experts who refused to believe, indeed, who had just committed the unforgivable sin. The sign they would get, nevertheless, was the sign of Jonah the prophet. Jesus indicated that the 3 days and nights Jonah spent in the belly of the great fish would be a picture of His death, burial, and resurrection from the dead. If they were not going to accept His miracles, or the words of the prophets fulfilled through Him, as evidence He was the Messiah, then the only sign their generation was going to get was the sign of His death, burial, and resurrection in the very same pattern as Jonah.