Here is a snippet from the letter:
We Theosophists believe that a religion is a natural incident in the life of man in his present stage of development; and that although, in rare cases, individuals may be born without the religious sentiment, a community must have a religion, that is to say, a uniting bond — under penalty of social decay and material annihilation. We believe that no religious doctrine can be more than an attempt to picture to our present limited understandings, in the terms of our terrestrial experiences, great cosmical and spiritual truths, which in our normal state of consciousness we vaguely sense, rather than actually perceive and comprehend; and a revelation, if it is to reveal anything, must necessarily conform to the same earthbound requirements of the human intellect. In our estimation, therefore, no religion can be absolutely true, and none can be absolutely false. A religion is true in proportion as it supplies the spiritual, moral and intellectual needs of the time, and helps the development of mankind in these respects. It is false in proportion as it hinders that development, and offends the spiritual, moral and intellectual portion of man's nature. And the transcendentally spiritual ideas of the ruling powers of the Universe entertained by an Oriental sage would be as false a religion for the African savage as the grovelling fetishism of the latter would be for the sage, although both views must necessarily be true in degree, for both represent the highest ideas attainable by the respective individuals of the same cosmico-spiritual facts, which can never be known in their reality by man while he remains but man.
Theosophists, therefore, are respectors of all the religions, and for the religious ethics of Jesus they have profound admiration. It could not be otherwise, for these teachings which have come down to us are the same as those of Theosophy. So far, therefore, as modern Christianity makes good its claim to be the practical religion taught by Jesus, Theosophists are with it heart and hand. So far as it goes contrary to those ethics, pure and simple, Theosophists are its opponents. Any Christian can, if he will, compare the Sermon on the Mount with the dogmas of his church, and the spirit that breathes in it, with the principles that animate this Christian civilization and govern his own life; and then he will be able to judge for himself how far the religion of Jesus enters into his Christianity, and how far, therefore, he and Theosophists are agreed. But professing Christians, especially the clergy, shrink from making this comparison. Like merchants who fear to find themselves bankrupt, they seem to dread the discovery of a discrepancy in their accounts which could not be made good by placing material assets as a set-off to spiritual liabilities. The comparison between the teachings of Jesus and the doctrines of the churches has, however, frequently been made — and often with great learning and critical acumen — both by those who would abolish Christianity and those who would reform it; and the aggregate result of these comparisons, as your Grace must be well aware, goes to prove that in almost every point the doctrines of the churches and the practices of Christians are in direct opposition to the teachings of Jesus.
I am also on board to be a helping contributor to an upcoming Theosophical wiki, which will be up at http://theosopher.net.