If you don't get it, re-read it until you do. The joke is in the last sentence:
Ambrose Bierce, The Devil's Dictionary:
PREDESTINATION, n. The doctrine that all things occur according to programme. This doctrine should not be confused with that of foreordination, which means that all things are programmed, but does not affirm their occurrence, that being only an implication from other doctrines by which this is entailed. The difference is great enough to have deluged Christendom with ink, to say nothing of the gore. With the distinction of the two doctrines kept well in mind, and a reverent belief in both, one may hope to escape perdition, if spared.