Barista comes to English from Italian, where a barista is a bartender. The Italian word derives from the English bar and the suffix -ista which comes from the Latin prefix -ista meaning one who practices something. A little like describing a musician who plays the cello as a ‘cellist’. or a person who rides a bicycle as a cyclist. Any word like economist, sexist, dentist, activist is derived from this root.
This Latin suffix is derived from Ancient, -ιστής (-istḗs), from -ίζω (-ízō) used to form verbs from nouns and adjectives, and the suffix τής (-tḗs) used to form agent nouns. Words in Ancient Greek that use this suffix include ἁγίζω (hagízō) meaning to make sacred or hallow, from the root ἅγιος (hágios) meaning holy, the origin of words such as hagiography, meaning writing to do with Saints. The Hagia Sophia in Istanbul is the church of Holy Wisdom.