WordNet sense 1 (of an obscure nature):

cryptic, deep, inscrutable, mysterious, mystifying

WordNet sense 2 (incapable of being explained or accounted for):

incomprehensible, inexplicable

WordNet sense 3 (having a secret or hidden meaning):

sibylline, cabalistic, kabbalistic, qabalistic

WordNet sense 4 (confined to and understandable by only an enlightened inner circle):

esoteric