Complex sentence: He abandoned his teaching profession in 1889 to be Blavatsky's private secretary and also became a joint-secretary of the Esoteric Section (E.S.) of the Theosophical Society, the E.S. was for those whom the Theosophical Society deemed more advanced.
Split into simpler sentences: He abandoned his teaching profession in 1889 to be Blavatsky's private secretary and also became a joint-secretary of the Esoteric Section (E.S.) of the Theosophical Society. The E.S. was for those whom the Theosophical Society deemed more advanced.