Eternity is what always is, the reality underlying all temporal phenomena, the nunc stans of St. Thomas Aquinas. It is vulgarly supposed to be an endless prolongation of Time, to begin in the future; it is instead the annihilation of Time, which is limited to this temporal world; in short, Eternity is the real Now.
“Eternity Exists, and All Things in Eternity” (Vision of the Last Judgment). Whatever was, is, and shall be is there. “Every thing exists & not one sigh nor smile nor tear, one hair nor particle of dust, not one can pass away” (Jerusalem). Nothing real can have a literal beginning. Man “pre-existed” before his creation in Eden, which was only his materialising, an episode of his Fall.