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.
Originally in English in reference to the sects that practiced adult baptism and arose in Germany from 1521. Probably so called because, as a new faith, they baptized converts who already had been baptized (as infants) in the older Catholic or older Protestant churches. Modern branches (notably Mennonites and Amish) baptize only once (adults) and do not actively seek converts. The name also was applied, usually opprobriously, to Baptists, perhaps due to the multiple immersions of their baptisms.
c. 1200, "to baptize into the Christian church," from Old English cristnian "to baptize," literally "to make Christian," from cristen "Christian" (see Christian). Especially to baptize and name as an infant, hence "give a name to at baptism" (mid-15c.) and the general sense of "give a name to" anything, without reference to baptism (1530s). Related: Christened; christening.