I don't know the full history of the term "Secret Doctrine." I did find this: 1833: Publication of vol. 1 of _Anacalypsis: An Attempt to Draw Aside the Veil of the Saitic Isis; or an Inquiry into the Origin of Languages, Nations, and Religions_ by Godfrey Higgins (vol. 2 – 1836). This is the book on which Helena Blavatsky based her first book, Isis Unveiled (1877), as noted by Leslie Shepard. He comments, “The plain fact is that Anacalypsis is the important prototype of the Theosophical framework. … Throughout his book Higgins insists on ‘a secret doctrine’ of esoteric knowledge guarded by priests.” [from The Book of Dzyan by Tim Maroney]. Blavatsky's work entitled _The Secret Doctrine_ was published in 1888 (the same year that the Golden Dawn was founded).
Anna Kingsford, who was the first president of the Theosophical Society in London, explained her understanding of the Secret Doctrine in the intro to her book _The Virgin of the World_:
"However various the manifestation of the universal consciousness, or being, whether as regards its different planes, or its different modes on the same plane, they all are according to one and the same law, which, by its uniformity, demonstrates the unity of the informing spirit, or mind, which subsists eternally and independently of any manifestation.
"For: “The Essence of all is One.”
"From the oneness of original Being comes, as a corollary, the law of correspondence between all planes, or spheres, of existence, in virtue of which the macrocosm is as the microcosm, the universal as the individual, the world as man, and man as God. “An earthly man,” says “The Key,” “is a mortal God, and the heavenly God is immortal man.” The same book, however, is careful to explain that by man is meant only those men who are possessed of the higher intelligence, or spiritual consciousness, and that to lack this is to be not yet man, but only the potentiality of man. It avoids also the error of anthropomorphism by defining Divinity to be, itself, neither life, nor mind, nor substance; but the cause of these.
"Ignorance of God is pronounced to be the greatest evil, but God is not to be discerned in phenomena, or with the outer eye. The quest must be made within oneself. In order to know [gnosis], man must first be. This is to say, he must have developed in himself the consciousness of all the planes, or spheres, of his fourfold nature, and become thereby wholly man. It is to his inmost and divine part, the spirit, that the mystery of existence appertains, since that is Pure Being, of which existence is the manifestation. … In such degree as man develops this consciousness he becomes an organon [intuition] of knowledge, capable pf obtaining certitude of truth, even the highest; and from being “agnostic” and incapable of knowledge, he becomes “Gnostic,” or has the Gnosis, which consists in the knowledge of himself and of God, and of the substantial identity of the two." [end of Kingsford quote]
Mary