Many people in history managed to attain spiritual revelation by studying the external parts of the Torah, the Mishnah, the Talmud, and not Kabbalah; that is, they used another language of presenting this wisdom. The entire Torah speaks about correction of a human being and that is the reason why the Torah was granted to us.
Texts of the Torah describe the states that one goes through during one’s advancement to the Creator, attaining adhesion with Him, and acquiring similarity of properties with Him. The problem is solely in the way of presenting it, meaning, in the language. Ways of presenting this material are not equal relative to the Light that Reforms; each of them possesses a different intensity of the Light. The most powerful Light is in the wisdom of Kabbalah.
They represent four layers, four filters that screen (hold) the Light on its way from top to bottom. The language of Kabbalah is pure Light. Kabbalah doesn’t limit the Light in any way, nor does it place filters on its way; Kabbalah doesn’t weaken the Light. Therefore, when one studies and denotes spiritual conceptions that refer to one’s soul, one elicits powerful Light that corrects him and brings him closer to the goal.
If one studies the Torah presented in any other language, then the same ideas, the same Light, goes through filters and descends to a person in an extremely weakened form. Consequently, the “quantity” of the Light is not enough to correct a person.
The four languages of the Torah: Pshat (simple explanation), Remez (hint), Drush (allegory) and Sod (secret) correspond to the worlds of Atzilut, Beria, Yetzira, and Assiya.
If one studies the wisdom of Kabbalah, it is the teaching (Torah) of the world of Atzilut, the Light that Reforms descends to him directly. If one studies any other way, then depending on the method he chooses, he receives less Light.
This is the way Kabbalah explains the division into the levels of Pshat-Remez-Drush-Sod, while the general opinion, is taken to the contrary, that Pshat (simple explanation), is the literal understanding, and Sod (secret), spiritual attainment.
From the 1st part of the Daily Kabbalah Lesson 11/20/12, “The Introduction to the Study of the Ten Sefirot”