"The analogy I've heard is that it's to know God exists and know His power, but to believe you can do better. If you're familiar with the story of the fall of Lucifer/Satan, it's essentially his actions in a nutshell: he knew God exists and he knew God's divine grace, yet he openly rejected it.
And thats why it's so unforgivable: God can't forgive you or save you if you reject His ability to do so."