The events in Mark 1:4-15 are a fulfillment of what Isaiah had prophesied. John was the "messenger" of verse 2 and "the one calling out in the wilderness" of verse 3.
Translators can make it explicit that John's coming was a fulfillment of Isaiah's prophecy. Alternate translation: "So John came" or "In fulfillment of that prophecy, John came" (See: [[rc://en/ta/man/jit/figs-explicit]])