What one unpardonable sin did Jesus teach about?

Jesus spoke of one very specific exhibition of gross blasphemy, and that is what He said was unforgivable. It was the sin of those Pharisees: closing one’s heart permanently against Christ even after the Holy Spirit has brought full conviction of the truth.

In effect, Jesus closed the door of heaven against these Pharisees who had so utterly and deliberately shut their hearts against Him. Why did He characterize their sin as blasphemy against the Holy Spirit? Because Jesus’ miracles were done in the power of the Holy Spirit. Even the Pharisees knew that in their hearts, yet they claimed He was operating in Satan’s power. In effect, they were calling the Holy Spirit the devil and giving the devil credit for what the Spirit of God had done.

What made this particular sin unpardonable was the finality of it. It was deliberate. It was an expression of coldhearted, determined unbelief. These Pharisees had seen, up close, more evidence than they could ever possibly need that Jesus was God incarnate. Their hearts were already settled. They would never believe, no matter what Jesus ever did or said. Therefore their sin was unforgivable.