The Torah says cryptically of Enoch: "he walked with God; and he was not, for God took him" (Genesis 5:24). Targum Pseudo-Jonathan tells us where he went.
"Hanok served in the truth before the Lord; and, behold, he was not with the sojourners of the earth; for he was withdrawn, and he ascended to the firmament by the Word before the Lord, and his name was called Metatron the Great Saphra."
Metatron. The Great Scribe. One of the most powerful angelic figures in all of Jewish mysticism, second only to God Himself in some mystical texts. In the Hekhalot literature and in later Kabbalah, Metatron is the heavenly scribe who records the deeds of humanity, the teacher of angels, the one who stands before the Throne of Glory. The Talmud (Chagigah 15a) and 3 Enoch expand on this: Enoch, the seventh generation from Adam, was taken alive into heaven and transformed into this angelic figure.
What makes a human ascend
The Targumist is careful about the verb. Enoch served in the truth. Before ascent, service. Before transformation, fidelity. He did not reach heaven by striving upward; he lived so truthfully on earth that earth could no longer contain him. Heaven took him the way a magnet pulls iron.
This is the Targumist's earliest hint of a tradition that will become enormous in Jewish mysticism: that a human being, by sufficient holiness, can cross the boundary into the angelic. Enoch is the prototype. The Tzaddik, in later thought, is a person whose life on earth has already begun to take on heavenly dimensions.