By the time of the building of the Second Temple, the keys to the Kabbala tradition had been entrusted to the last prophets of the Jewish people as well as to its greatest sages. Together, they constituted the 120 Men of the Great Assembly. It was this body of sages that formulated the Mishna in Tractate Chagigah, stating: "The Maaseh Merkava may be taught only to individual students (one at a time), and they must be wise, understanding with their own knowledge." They thus insured the continued transmission of the Kabbala tradition by restricting its practices to the smallest possible circle of masters. Outside of this circle these practices would remain almost totally unknown. This policy continued until after the destruction of the Second Temple in 70 C.E. It was only then that things began to change.
After the Bar Kochba revolt in 132-135 C.E., things took a drastic turn for the worse. At this time the Romans became intent on uprooting the last traces of Torah from the Jewish people. The Hadrianic persecutions reached such a crescendo that all teachers of Torah were condemned to death. This was the time of the Ten Martyrs, among who were the final transmitters of the Kabbala tradition.
As a result of these persecutions, the oral tradition from Sinai, in particular the Kabbala tradition, was in danger of being forgotten. At this time, Rabbi Akiva (50-135 C.E.) received his tradition. He was considered the greatest sage of his generation, a master of the revealed Torah, as well as the concealed. Rabbi Akiva possessed the Merkava (Chariot) tradition. Many sources attribute to Rabbi Akiva the authorship of the Sefer Yetzira (Book of Formation), one of the oldest and most obscure Kabbalistic texts. Just as the great bulk of the Talmud bears Rabbi Akiva's stamp, so does the Sefer Yetzira. It was Rabbi Akiva who transmitted these teachings in a well-defined form.
At this time, Rabbi Nehunia ben Hakanah and his disciple, the High Priest Rabbi Yishmael ben Elisha, put into writing the Sefer Bahir (Book of Illumination) and the Pirkey Hekhalot Rabatai (The Greater Book of the Divine Chambers). These two sages redacted the traditions they had received in order to preserve them from oblivion during the violent times in which they lived. Commenting on the Mishnah quoted above, Rashi (Rabbi Shlomo Yitzchaki-Yarchi 1040-1105) asserts that one of the main texts for the study of Maaseh Merkava was the Pirkei Hekhalot Rabatai. This text contains actual meditative exercises, disciplines, and directions for entering the prophetic state. PULL The Romans were actually killing all the great teachers, the sole transmitters of the revealed and concealed traditions…
The Zohar (Book of Splendor), one of the main pillars of Kabbala, was taught by Rabbi Shimon bar Yochai around 135 C.E. Rabbi Shimon also lived during these tumultuous times when the Romans were actually killing all the great teachers, the sole transmitters of the revealed and concealed traditions. During the thirteen years that Rabbi Shimon spent hiding in a cave with his disciples, he taught what he had received from his teachers. During this time as well, he received Divine Inspiration ( Ruach HaKodesh) and merited the revelation of Elijah the prophet. There was a tradition that if the Oral Torah was endangered to the point of being forgotten, it was permitted to put it into writing. Therefore, all the masters, Rabbi Akiva, Rabbi Nehunia, Rabbi Yishmael, and Rabbi Shimon, set a precedent. They began committing parts of the oral tradition to writing. Despite this, however, all of these texts were not given final form for several generations after these teachers. This would be the task of their disciples.
All of these major texts, the Yitzira, the Bahir, the Hekhalot Rabatai, the Zohar, and the various parts of the revealed Torah, contained the basic teachings which had been passed down through the prophets and sages from Moses. The time had come to commit these teachings to writing. Interestingly enough, all of these works are obscure to the point of begging the question: what was gained? What had been written down remained, as the Torah had been in its time, a closed book. The keys were to remain oral. Just enough had been written down to insure that only someone familiar with the tradition would understand. This whole corpus of writings, ranging from the practical understanding of the commandments to the most sublime experience of prophecy, remained a closed book. Yet, the Kabbala tradition had been saved.