Now that we know some of the context behind this passage, I think it’s important to define some things. Blasphemy means to disrespect God greatly or denounce God to something repellant or disgusting. In this case, you can see how the Pharisees did this to Jesus and the Holy Spirit by their accusation by saying He was using evil powers.
Speaking of the Holy Spirit, who is the Holy Spirit? And why is speaking against the Holy Spirit such a big deal? The Holy Spirit is very important because He secures our salvation and reminds us of Jesus’ teachings that are essential for salvation. Basically, without the Holy Spirit, salvation wouldn’t be obtainable.
But why is blaspheming against Jesus forgivable, but not the Holy Spirit? The main difference is in the Holy Spirit’s role. Basically, if you reject the Holy Spirit, salvation is unobtainable, not because God isn’t willing to grant you forgiveness and mercy, but because you closed your own heart off to the only thing that can convict you to repent. Blaspheming against the Holy Spirit is basically just continuous, unrepented sin.