Jesus Is The Gospel.

In Mark 1:14, John the Baptist is dismissed from the narrative with a reference to his being arrested. Mark mentions the arrest to close the preaching of John; he wants to move on to his real topic of interest: Jesus and the heralding of the gospel. When he introduced John in verse 4, he omitted the term “gospel,” suggesting that he is reserving it exclusively for the one who is coming after John, that is, Jesus.
Mark will come back to John and tell us the circumstances surrounding his arrest and eventual death, but even then it’s not for the purpose of advancing the narrative. John is only functional in the gospel in how he relates to Jesus, who is the content of the gospel. John is the forerunner. What he says may be the same as what Jesus says - and, later, Matthew will underscore this by making the words of Jesus correspond to the words of John verbatim - but in the gospel of Mark, the sole reference is Jesus, the Christ. For him, Jesus is the gospel.  

Notes:
Jeremiah 40:12; 44:28
Mark 6:30
Galatians 1:11-12
Ephesians 4:20
κηρύσσων (kérussó) - to herald, proclaim
λέγων  (legò) - to say, to speak
καιρὸς (kairos) - time
χρόνος (chronos) - time
μετανοέω (metanoeó) - to repent, to change one’s mind
שׁוּב - to return
πιστεύω (pisteuó) - to believe, to trust
Tarazi, Paul Nadim: New Testament An Introduction vol. 4 - Matthew and the Canon (SVS Press, 2009)

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Jesus Is The Gospel.
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