Suggesting that the truth has somehow been totally realized and frozen into place for eternity at a given point in that process is nothing short of arrogance. At a very basic level, Spong's critique appears as a threat to those concerns in the minds of his critics. Hence the ferocity of their ad hominem attack here. Jack Spong seeks to spark a discussion of the faith in a hope to revive it, to enliven it, to bring others into the conversation. A better guard for the faith could hardly be found than one who seeks to ensure its very life.
Fear is a powerful motivator as is the drive for power and control.
I pray for our church in the hands of such fearful self-limited men. One last comment. Cognitive dissonance is the painful phenomenon that arises when one's truth comes into conflict with another irreconcilable truth. One's answers no longer meet the questions, essentially. One of the ways to deal with this is denial.