The basic information of Jewish mysticism is readily available online and in print today. Yet kabbalah continues to be treated as knowledge that is secret and is only appropriate for people to learn after they have reached a certain age and/or attained a certain level of learning. I'd like to understand what the reason is for the continued secrecy.
I heard Rabbi Akiva Tatz explain as follows - People think that "mysticism" is a secret means that you don't know it because no one will tell you. But once you qualify, you'll find an old rabbi in a back alley who will let you in on the secret. But that isn't what it means that "mysticism" is secret - it means that it is by nature not possible to tell. It is information which cannot be expressed directly from one person to another. It is only after you have enough of your own understanding that someone can lead you to figure it out on your own. But it simply isn't possible without the prerequisite training - the words are empty words, and are at best meaningless.
As the Mishna in Chagiga 11b says:
ולא במרכבה ביחיד אלא אם כן היה חכם ומבין מדעתו
We do not expound the maaseh merkava even to an individual, unless he is a wise man who understands it from his own awareness
it is written in a special code (metaphorical) language so that only those with the necessary introductions and level of torah scholarship can understand it.
so even with all the literature out there, it remains incomprehensible.
see this audio by Rabbi Yaakov Hillel (regarded as one of the top kabalists today)
The point of hiding it is that it shouldn't end up looking the way it does today. The Rambam in his introduction to Mishnayos explains that when those who aren't meant for deep stuff get hold of it they mock it. This is what we see today.
If only the secrets would have remained secrets it would have still been admired by those who don't relate to it. In a way, it is considered understanding when you at least comprehend how far it is from you. Even if you can't climb the mountain if you see how high it is and you realize how far you are from getting there you are more on target than someone who imagines he is there already.
My teacher, HaRav Aryeh Kaplan (of most Blessed Memory) , used to tell me that when I would be ready for him, my Maggid would appear and lead me further.
It was further explained in great detail to me that only truly pious and wise men would ever reach even the most bottom rung of Kabbalah.
In fact Kabbalah requires 24/7 study and devotion to Torah. It can never be achieved by the non-Jew, and not even then unless one is worthy of it.
For those brave enough to try, I suggest Rav Kaplan's "Meditation and Kabbalah" wherein, amongst other things, he discusses the The Talmudic Mystics, Schools, Safed and the Ari, and the Hasidim.
and/or his "Sefer Yetzirah" translation, of the GRA version, wherein he discusses in great detail "The Creation".
A better than average knowledge of the Hebrew Language is ESSENTIAL.
Up-front warning. This is not going to be a popular answer. This is a halakhicaly acceptable opinion, but it is not the dominant hashkafah, and doubting kabbalah raises many hackles. Having said that:
The real answer is that it was never secretive. The source books were published and widely spread in the same decade as when they were written. The Bahir, Zohar, and Es Hayyim were very rapidly disseminated. Mystical schools were present in every city kabbalah had any major presence, and it was taught on such an individual level that prayer books were altered to accommodate kabbalistic teachings. Kabbalah presents itself as secretive for at least three reasons. The first is because it borrows imagery heavily from the gnostic tradition, which in turn, has its sources in Greek mystery traditions. Kabbalah also needed to present itself as secretive to explain why it sprang up so rapidly after not having any apparent mesorah and in contradiction to earlier rabbinic teachings. By saying it was secretive, the early mekabbulim could say that its absence of mesorah is only apparent, and the kadmonim were unaware of the core truths of Judaism or purposefully taught falsehoods to avoid teaching the kabbalah (H"V). The third major reason kabbalah presents itself as secretive is that it purports to be the maaseh bereshith and maaseh merkqavah mentioned in the talmud as being forbidden to widely teach.