For me, its a toss up between this passage:
Rev 20:12,15 And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works...And whosoever was not found written in the book of life was cast into the lake of fire.
...and the Matt. 7:21-23 passage already mentioned. The thought of Jesus looking into my eyes, saying He never knew me, and then commanding me to depart from Him is a scary thought.
As you said, "May all of us give diligence to "make our calling an election sure".