(AG-uh-thist): n. from Greek-derived agath- (good): a person who believes all things tend toward ultimate good.
Sometimes I memorize new words by remembering clues. When I first came across this word I thought of Agatha Christie. If you don't know who Agatha Christie is, you should. Anyway, then I thought, "It's good to solve crimes. Therefore Agatha = Agathist". My next thought was Agathist reminds me of Optimist, and they have similar meaning. If that doesn't work, think of one bubbly hopeful person you know in your life and repeat. "********_ is such an agathist" 10x.