"initiatory sacrament of the Christian faith, consisting in immersion in or application of water by an authorized administrator," c. 1300, bapteme, from Old French batesme, bapteme "baptism" (11c., Modern French baptême), from Latin baptismus, from Greek baptismos, noun of action from baptizein (see baptize). The -s- was restored in late 14c.
The signification, qualifications, and methods of administration have been much debated. Figurative sense "any ceremonial ablution as a sign of purification, dedication, etc." is from late 14c. Old English used fulluht in this sense (John the Baptist was Iohannes se Fulluhtere).
Phrase baptism of fire "a soldier's first experience of battle" (1857) translates French baptême de feu; the phrase originally was ecclesiastical Greek baptisma pyros and meant "the grace of the Holy Spirit as imparted through baptism;" later it was used of martyrdom, especially by burning.
word-forming element making nouns meaning "place for, art of, condition of, quantity of," from Middle English -erie, from Latin -arius (see -ary). Also sometimes in modern colloquial use "the collectivity of" or "an example of."
<a href="https://www.etymonline.com/word/baptistry">Etymology of baptistry by etymonline</a>
Harper, D. (n.d.). Etymology of baptistry. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/baptistry