Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
In reading this verse, we usually jump to the second part and ask ourselves what the unpardonable sin may be. In doing this, we miss what Jesus says in the first part, namely that there is a great desire on God’s part to forgive.
The problem is that forgiveness is not easy and comes at infinite cost, that is, Christ’s sacrifice on the cross in our place. In resisting the Holy Spirit we may place ourselves outside of God’s forgiveness. It is possible to place ourselves outside of God’s infinite willingness to forgive. Scary.
-Paraphrasing Tim Keller in The Sin Against the Holy Spirit