of the essence

Of the greatest importance, crucial, as in Time is of the essence. This idiom, which uses essence in the sense of "the most important element of something," was first recorded in 1873, although the phrase the essence of ... was already being used in the mid-1600s.

The adjective has one meaning:

Meaning #1: of the greatest importance
  Synonyms: all-important, all important, crucial, essential