proselytize (v.)

"one who changes from one sect, creed, etc. to another," late 14c., proselite, "a convert, especially "a heathen convert to Judaism" (in Biblical writings, e.g. Matthew xxiii.15, Ezekiel xiv.7), from Old French proselite (13c., Modern French prosélyte), from Late Latin proselytus, from Greek prosēlytos "convert (to Judaism), stranger," literally "one who has come over."

It is a noun use of an adjective meaning "having arrived," from pros "from, forth, toward" (see pros-) + eleusomai "to go, come" (from PIE *elu-to-, from root *leudh- "to grow up, come out" (see liberal (adj.)).

word-forming element used to make verbs, Middle English -isen, from Old French -iser/-izer, from Late Latin -izare, from Greek -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached.

The variation of -ize and -ise began in Old French and Middle English, perhaps aided by a few words (such as surprise, see below) where the ending is French or Latin, not Greek. With the classical revival, English partially reverted to the correct Greek -z- spelling from late 16c. But the 1694 edition of the authoritative French Academy dictionary standardized the spellings as -s-, which influenced English.

In Britain, despite the opposition to it (at least formerly) of OED, Encyclopaedia Britannica, the Times of London, and Fowler, -ise remains dominant. Fowler thinks this is to avoid the difficulty of remembering the short list of common words not from Greek which must be spelled with an -s- (such as advertise, devise, surprise). American English has always favored -ize. The spelling variation involves about 200 English verbs.