Brahma (Hiranyagarbha) knows this, which is hidden in the Upanishads, which are hidden in the Vedas, as the Brahma-germ. The ancient gods and poets who knew it, they became it and were immortal.

In plain words: This truth is hidden in the Upanishads, which are hidden inside the Vedas. Brahma, the first created being, knows it as the seed of all. The ancient gods and poets who knew it became one with it and became immortal.