The words “esoteric” and “essential” have distinct meanings, and finding synonyms for them can add variety and nuance to your language.
For “esoteric”, some synonyms could be “arcane”, “recondite”, and “obscure”. “Arcane” implies something mysterious and known only to a few. For example, “The arcane rituals of the ancient cult were a source of fascination.” “Recondite” suggests something that is difficult to understand because it is profound or abstruse. For instance, “His recondite theories on quantum physics left many scratching their heads.” “Obscure” can mean hidden or not easily understood. For example, “The obscure reference in the poem puzzled the scholars.”
For “essential”, synonyms could include “vital”, “crucial”, and “indispensable”. “Vital” emphasizes something that is necessary for life or for a particular purpose. For example, “A good diet is vital for maintaining health.” “Crucial” indicates something that is extremely important and decisive. For instance, “The crucial decision had to be made quickly.” “Indispensable” means something that cannot be done without. For example, “Water is indispensable for survival.”