Our Gemara on Amud Beis says that words of Torah are to be hidden. What does that mean, why should Torah be hidden? To the contrary, Moshe says to Yehoshua (Bamidbar 11:29):

וַיֹּ֤אמֶר לוֹ֙ מֹשֶׁ֔ה הַֽמְקַנֵּ֥א אַתָּ֖ה לִ֑י וּמִ֨י יִתֵּ֜ן כׇּל־עַ֤ם יְהֹוָה֙ נְבִיאִ֔ים כִּי־יִתֵּ֧ן יְהֹוָ֛ה אֶת־רוּח֖וֹ עֲלֵיהֶֽם׃

⁦But Moses said to him, “Are you jealous on my account? Would it only be granted that all the LORD’s people were prophets, that the LORD put His spirit upon them!”

And the Gemara (Nedarim 37a) tells us that Hashem commands that Torah study to be free of charge:

“Just as I teach you for free, without payment, so too you also shall teach for free.”

So what is being hidden? Rashi and Meiri seem to understand that hidden is not the same as secret. That is, Torah should be taught in quiet secluded places out of respect for the material, but not necessarily secret.

There is, however, absolutely a secret side to the Torah, and as I shall demonstrate, it’s the tip of the iceberg. That is, the LARGER portion of the Torah is hidden.

Based on an interpretation of the verses in Isaiah (3:1-4), The Gemara (Chaggigah 14a), describes Torah as given in a whisper. The Gemara (Succah 28a) describes a “small matter” as the halakhic analyses of Abaye and Rava, and the “great matter״ as the “Works of the Heavenly Chariot.” 

The Mishna Chaggigah 2:1 says that one is not even allowed to teach this esoteric part of Torah, even privately to an individual, unless he has the wisdom to infer it by himself and the teacher only provides “chapter headings”. Chapter headings presumably means the basic ideas and not the details.

There is a disagreement between the Maimonidean camp and the mystics about what מעשה מרכבה is. According to Rambam it’s about the true nature of God, angels and the soul, essentially metaphysics. According to the mystics, it’s the various sefirotic realms and how their emanations enter into this world (See Bartenura and Rambam commentary on the Mishna.) Tiferes Yisrael (on Mishna Chaggigah 2:1) understands it to include practical Kabbalah, that is the theurgic use of angelic names and entities to influence reality.

But why is this material kept hidden and secret? Tiferes Yisrael (ibid) says because the material is so intensely holy, it must be honored by keeping it exclusive and secret. He adds, and the Rambam states it similarly, these concepts are impossible to fully verbalize and thus explaining them will confuse and obfuscate instead of clarify. This helps us understand why the student must be wise and capable of inferring the subject matter from mere chapter headings. It’s not because the teacher doesn’t want to teach more, but because the teacher CANNOT teach more. Instead of teaching, the rebbe has to facilitate learning and understanding by setting a correct frame.

Living in an era of unprecedented access to information, it seems odd and alien that knowledge would be deliberately restricted. It is even more odd that parts of the Torah, perhaps even the “Great Matters” are not even allowed to be taught explicitly. Yet it is not as odd as it seems. Consider that even in regard to secular law, the real reasons, morality and legal theory stated behind the law is not stated. Perhaps in studying case law, which is essentially the secular equivalent of Gemara, more of the rationales are discussed. But even that doesn’t get into the nitty gritty of legal theory and morality.

While we are on the topic of morality, the Chovos Halevavos in his introduction makes a fascinating observation. In his process of wondering why there were so few treatises that discuss the inner duties and obligations of the heart, he notes an interesting verse. The verse in Deuteronomy (17:8) states:

כִּ֣י יִפָּלֵא֩ מִמְּךָ֨ דָבָ֜ר לַמִּשְׁפָּ֗ט בֵּֽין־דָּ֨ם ׀ לְדָ֜ם בֵּֽין־דִּ֣ין לְדִ֗ין וּבֵ֥ין נֶ֙גַע֙ לָנֶ֔גַע דִּבְרֵ֥י רִיבֹ֖ת בִּשְׁעָרֶ֑יךָ וְקַמְתָּ֣ וְעָלִ֔יתָ אֶ֨ל־הַמָּק֔וֹם אֲשֶׁ֥ר יִבְחַ֛ר יְהֹוָ֥ה אֱלֹהֶ֖יךָ בּֽוֹ׃

⁦If a case is too baffling for you to decide, be it a controversy over homicide, civil law, or assault—matters of dispute in your courts—you shall promptly repair to the place that the LORD your God will have chosen.

Essentially the Torah says if you do not understand a halakha, you should go to the Great Sanhedrin in Jerusalem, and they will guide you.

Chovos Halevavos asks, “How come it does not state questions regarding Love thy neighbor, how to pray, or what to believe?” The answer is, these matters are too close to the heart and it is incumbent upon you to find the truth inside yourself. Wow, sometimes it is amazing how the most profound statements are hiding in plain sight, in the classic rishonim.  In a way, he really is saying something similar to what we saw earlier, there are matters that words fail to accurately express.  Perhaps this is part of what is called, “The Fifth Volume of Shulkhan Arukh”.  (There is NO REAL fifth volume, and this colloquially refers to common sense.)

Furthermore, in chapter 10 of of Shaar HaYichud, Chovos Halevavos equates three disparate concepts under the general heading of matters of Torah that cannot be easily described or written explicitly, for the general masses will misunderstand them and become confused:

If the scriptures had employed more accurate, truer terminology, then nobody would have understood it except the wise, understanding reader and most of mankind would have been left without religion and without Torah (guidance) due to their limited intellect and weak understanding in spiritual matters. But the [literal] word which may be understood in a material sense will not damage the understanding person because he recognizes its real meaning, and it is at the same time beneficial to the simple person so that it will fix in his heart and mind that there is a Creator which it is his duty to serve.

Likewise we will say for all subtle matters found in the Torah such as the reward in the next world or its punishment. 

And likewise we will say for the clarification of the inner wisdom (the duties of the heart) which was our intention to clarify in this book. The Torah was very brief in expounding their matters, relying on the intelligent men. The Torah only hinted at it to arouse one on it, such as mentioned in the Introduction of this book, so that anyone who is able to enquire and investigate them will be aroused to do so until he has understood and mastered them as written: "those who seek G-d will understand all things" (Mishlei 28:5).

Incredibly, Chovos Halevavos is equating the duties and obligations of the heart to other parts of the Torah that are mysterious and secret. Meaning that it simply is something that must be learned and developed with consciousness instead of merely taught.

I will conclude with the following thought. The concept that parts of the Torah are meant to be hidden, especially the mystical part, is an almost universal human instinct to keep mystical experiences within a secret lore or society. This spans the Greeks, with the ancient secret society Pythagoras, Hindu, Native American, Egyptian and Chinese Daoism. ( https://www.britannica.com/topic/mysticism/Mysticism-and-secrecy ). Incredibly, the Celtic mystics even had a prohibition against writing down their Oral “Torah” (See Mircea Eliade, A history of religious ideas volume two, page 140 ) The point is that if this is so universal, at the minimum it indicates a deep psychological intuition which says such experiences must be private in order to properly experience them. It also possibly indicates a basic mystical tradition that goes back to times of Adam and Noah. This is especially so when certain ideas cross cultures such as afterlife, reincarnation and the soul’s knowledge of all wisdom prior to birth (Compare Gemara Niddah 30b to Meno (82b–85b), which was written by Plato. Both Greek philosophical tradition and Jewish tradition believe that our souls know all of wisdom prior to birth.)