What might that be?
The Bible speaks of one unpardonable sin. Jesus talked about a blasphemy against the Holy Spirit that can never be forgiven (Mt. 12:31-32; Mk. 3:28-29).
The unpardonable sin, IMO, is that you DO NOT mock God. Mock anything else you wish, but not God. You can tease God, IMO, and make light-hearted jokes at his expense, but never, ever, mock him with serious intent.
Tuor