A preacher was the title given to a person on Earth who spread the Word of God.
In a vision sent to Benjamin Sisko by the Prophets and a Pah-wraith, Benny Russell often encountered an African-American preacher in Harlem, New York. (DS9: "Far Beyond the Stars")
In the revised story outline of TOS: "Mudd's Women" (dated 23 July 1964), preachers stationed at colonies were implied as being commonplace in the future envisioned in Star Trek.