I promise to be brief.
“Baptism” is mentioned in the beginning and end of Mark’s Gospel.
Mark first tells the reader about John’s baptism in 1:4-13. He references John’s prophetic call (“The beginning of the gospel about Jesus Christ,” 1:1-3), how baptism fulfills it (vv4-6), and his message:
“After me will come one more powerful than I…” in reference to Jesus (v7). John was, after all, the forerunner for the Messiah.