You may already know this, but even if you do, here is a fun summary of why our English translations use both of these words. The title “John the Baptist” (sometimes translated “John the Baptizer”) comes from the Greek ho baptistēs, meaning “the one who baptizes,” and refers to John’s ministry of baptizing people in the Jordan River as part of his call to repentance. It has no connection to the Protestant “Baptist” denomination, which arose over 1,500 years later in the 17th century. Interestingly, the New Testament uses both forms, similar to how English might use “cyclist” and “biker.” The noun, as in “John the Baptist” appears in Matthew 3:1, Matthew 11:11, and Luke 7:20, while the participle as in “John the Baptizer” (Greek ho baptizōn) occurs in Mark 1:4, Mark 6:14, Mark 6:24 (See, for example, the RSV translation). So there really are two ways of naming John. Both word forms simply describe John’s role as the one baptizing people in water. Here is a link showing how two different Greek words are being translated in same way in the ESV (You can click on the word to peak behind the English to see the Greek differences).
Adorn the Gospel 