A Textbook of Theosophy – C.W. Leadbeater (1912)

We often speak of Theosophy as not in itself a religion, but the truth which lies behind all religions alike. That is so; yet, from another point of view, we may surely say that it is at once a philosophy, because it puts plainly before us an explanation of the scheme of evolution of both the souls and the bodies contained, in our solar system. It is a religion in so far as, having shown us the course of ordinary evolution, it also puts before us and advises a method of shortening that course, so that by conscious effort we may progress more directly towards the goal.

Isis Unveiled by H. P. Blavatsky-1877

The book is written in all sincerity. It is meant to do even justice, and to speak the truth alike without malice or prejudice. But it shows neither mercy for enthroned error, nor reverence for usurped authority. It demands for a spoliated past, that credit for its achievements which has been too long withheld. It calls for a restitution of borrowed robes, and the vindication of calumniated but glorious reputations. Toward no form of worship, no religious faith, no scientific hypothesis has its criticism been directed in any other spirit. Men and parties, sects and schools are but the mere ephemera of the world's day. Truth, high-seated upon its rock of adamant, is alone eternal and supreme.

A Rough Outline of Theosophy: Annie Besant 1921

Hence the doctrine of Reincarnation is the very core and essence of Theosophy, and according to the hold this belief has on life, so will be the grasp of the learner on all Theosophic truth. Through the physical body, including its brain, the reincarnated Ego has to work for the term of that incarnation, and thus it dwells in a tabernacle of its own construction, the inevitable resultant of its own past earth lives.