【Recondite】 , 【abstruse】 ,  【occult】 ,  【esoteric】  can all mean being beyond the power of the average intelligence to grasp or understand.

【Recondite】 stresses difficulty resulting from the profundity of the subject matter or its remoteness from ordinary human interest. It often, especially as applied to persons, implies scholarly research carried beyond the bounds of apparent usefulness.

【Abstruse】 suggests extreme complexity or abstractness in the material as well as its remoteness from the ordinary range of human experience or interest.

【Occult】 basically implies secret, mysterious knowledge purporting to be attainable only through special and often supernatural or magical agencies and not through ordinary channels of human reason.

But often the word is used with much weakened force to mean little more than mysterious.

【Esoteric】 basically implies knowledge guarded by, and imparted only to, members of a cult or inner circle of initiates, but it is extended in general use to describe knowledge in the possession only of adepts, students, and specialists.