Essays has a Search tab, third alongside Recent and Collections. It searches the full text of every essay on the site, not just titles and descriptions.
Type more than one word and every word has to appear somewhere in the essay for it to match. Grace mercy will only return essays containing both words, not either one. Wrap a phrase in quotation marks, like “grace not fair”, to search for those words together and in that order, instead of scattered anywhere in the piece.
Each result shows the sentence where your term actually appears, with the match highlighted, so you can tell why an essay matched before you click into it. If you remember a phrase from something you read once but not which essay it was in, this is the fastest way back to it.