Best "What The Unpardonable Sin Really Is" (It Is Refusing To Allow Your Heart To Love Others As The Holy Spirit Would Have You)

From the book Your God Is Too Small by J. B. Phillips (1961, New York: The Macmillian Company, page 105 [page 76 of 92 in linked pdf]):

"This, from an examination of the context, would appear to be a combination of refusing to recognize truth of the oneness of God and refusing to allow the heart to love others."

The scribes and Pharisees were refusing to allow their hearts to love others as the Holy Spirit would have them. Do not do that. Allow your heart to love.