Why does there seem to be no evidence that Jesus passed on to his apostles at least some of the main spiritual elements such as karma, reincarnation, and the techniques of the spiritual path that are so well-established in Buddhism and Hinduism? When one considers his level of spiritual development, it doesn’t seem likely that he wouldn’t be aware of them. And if he was, why would he not clearly explain those elements to his apostles to pass on to other people?
Thank you for your thoughtful question. Actually there are quite a few references to karma and reincarnation still in the Bible, both in the Old and New Testaments.
I say "still" because it is fairly well documented that at the first major Church council (the Council of Nicaea in A.D. 325) it was decided to "purge" the scriptures of any references to karma and reincarnation.
The idea was that Christians would be better off to think that they have only one life to live and therefore only one chance at salvation through the Church and it’s teachings.
But the Council wasn’t quite thorough enough in this process, and many passages in our present Bible still hint at these ideas. Rather than go into them in detail, I will refer you to a book which explains all this quite well.
Revelations of Christ was written by Swami Kriyananda, and is available from Ananda’s publishing house, Crystal Clarity. I think you'll find all the answers you need and a lot more. It’s fascinating!
This book will also answer your second question. Paramhansa Yogananda, in deep communion with Jesus Christ, did clearly say that Jesus "...taught Kriya Yoga, or something very similar to it, to his closest disciples.
There are other references in the Bible which also hint at meditation experiences. "Be still and know that I am God." "If therefore thine eye be single, thy whole body shall be full of light." "I die daily," and so on.
Why is it not clearer? The life and mission of Jesus took place in the depths of Kali Yuga — a very dark time for our planet. He taught in parables and in ways that average people could comprehend. And again, perhaps much as been lost from the scriptures, over the many years since they were written.
All great masters seem to do this, at least to some extent — that is, teach appropriately to the times they live in and keep the deeper teachings somewhat hidden, except from those who are better able to understand and benefit from them.