baptize, baptise XIII. — OF. baptisier (mod. baptiser) — ecclL. baptizāre — Gr. baptízein, f. báptein dip; see -IZE.
So baptism XIV. ME. baptesme — OF. baptesme (mod. baptême) — ecclL. baptismus — ecclGr. baptismós. Baptist name of John, forerunner of Jesus Christ. XII. — (O)F. baptiste — ecclL. baptista — ecclGr. baptistḗs.

bap·tize / ˈbapˌtīz; bapˈtīz/ • v. [tr.] administer baptism to (someone); christen: he was baptized Joshua. ∎  admit (someone) into a specified church by baptism: Mark had been baptized a Catholic. ∎  give a name or nickname to: he baptized the science of narrative “narratology.”