There was a time, the sages taught, when the Divine Name of twelve letters was taught openly to anyone who came to learn. A student could carry it home the way he carried any other piece of Torah.
Then the antinomians began to multiply — those who used holy knowledge for unholy ends — and the teachers grew cautious. The name was entrusted only to the most discreet among the priestly families, and even they began to swallow it. During the priestly blessing in the Temple, one priest would pronounce the Name so rapidly, so softly, that the congregation's own singing covered the sound. Rashi, centuries later, admits that by his own day no one remembered what the twelve letters had been.
Once, Rabbi Tarphon told the story this way: "I was standing in the Temple courts, close to the high priest. While the other priests were blessing the people, I leaned in and listened very carefully, and I heard the high priest pronounce the hidden Name." He had pressed his ear against a door that only the discreet are allowed to open.
The lesson is not that sacred knowledge must be hoarded, but that not every tongue is ready to carry it. A Name too powerful for a careless heart is protected by silence, not by lock and key.
(From the 1901 Hebraic Literature anthology, drawing on Kiddushin 71a.)