Titles of books are sometimes indications only. Here for instance, there is not really any doctrine, but just reflections of the moon in a puddle. And quite certainly there is no teacher. Essential understanding, however, might have found its way into occasional pages - whether understood or not by the transmitter is beside the point.
Under the title of this book only three religions are cited, those that are formally non-dualist. But this apparent limitation does not imply that such is not also the essential 'doctrine' of the three Semitic faiths, Judaism, Christianity and Islam, which are formally dualist, and whose esoteric aspects are Kabala, Gnosis and Sufism.
In Christianity the dualism of Creator and created is resolved in what is implied by Godhead, but this is not developed in the theology, moreover the recorded words of Jesus are few and are chiefly addressed to the simple-minded, and the esoteric doctrine was cast out by the council of Constantinople in A.D. 553. Therefore the Christian evidence chiefly resides in Gnostic records that are little known, in the early Fathers, and in sages and saints such as Meister Eckhart and St. John of the Cross, who were obliged by the dogmas of the Church to cloak the non-dualism which is implicit in their realisation of the truth. For this reason it is unpractical to use Christian evidence in such a collection of observations as this.
It seems unlikely that anything but the superficial teaching of Jesus, that which he taught in parables 'so that they should not understand', has been available to the 'Christian' public since the excommunication of Origen in A.D. 553, three hundred years after he wrote his works.
However, in view of the tidal-wave of interest in metaphysics which reveals a considerable percentage of modern man as being driven to seek the truth concerning himself and the universe, it seems inevitable that the day will arrive when the doctrines of Iesous Christos will once more be revealed to mankind.
I have not mentioned Tao? That which is understood does not need statement. The doctrine of Tao is an implicit rather than an explicit doctrine. The doctrine of Tao is itself the essential doctrine. This little book might have been called Tao - were not such a title presumptuous.
In general, capital letters have been used when the term implies that which is unique. When the same word is written with a lower-case letter multiplicity is implied.
