Quite often, simple questions are rarely answered. Why, for example, does Mark's Gospel begin with John the Baptist, not with Jesus himself? - Here another glimpse into my current project on Marcion's Gospel: A Synoptic Commentary (and please feel free to comment):
R. Alan Cole in his Commentary on Mark points out that the Gospel ‘stands in direct relation to God’s whole revelation to his people in the past’, so that ‘the New Testament is to Mark not a breach with the Old Testament, but a fulfilment of it’. Now, why would one need to point out the link between new and old, since Christians of the first decades have been Jews? Paul did not preach a breach with the old covenant, although he was quite critical of his own people and belief. Since when do we have the notion of a ‘New Testament’ that is contrasted with an ‘Old’ Testaments, so that someone like Mark would need to bridge a potential breach? If we trust my colleague Wolfram Kinzig, it was Marcion of Sinope who, for the first time, called the collection of Christian writings ‘New Testament’ and set it out as an antithesis to a Jewish ‘Old Testament’. Mark's opening, therefore, would fit the anti-Marcionite stance of an author who would like to retain the link between Christianity and Judaism, a connection which was called into question by Marcion. But let us compare the opening of Marcion’s Gospel and that of Mark in more detail:
 (Tyndale New Testament Commentaries II, 2nd ed. 1989), 105-6.
 ‘Καινὴ διαθήκη’: JTS N.S. 45 (1994): 519-44.
… the delightful message [Gospel] of
‘O wonderful wonder, delight, power and astonishment we find in faith what is unspeakable, beyond thinking and uncomparable’
1:1 The beginning of the delightful message of Jesus Christ, the Son of God. 1:2 As it is written in Isaiah the prophet,
“Look, I am sending my messenger ahead of you, who will prepare your way, 1:3 the voice of one shouting in the wilderness, ‘Prepare the way for the Lord, make his paths straight.’”
1:4 In the wilderness John the baptizer began preaching a baptism of repentance for the forgiveness of sins. 1:5 People from the whole Judean countryside and all of Jerusalem were going out to him, and he was baptizing them in the Jordan River as they confessed their sins. 1:6 John wore a garment made of camel’s hair with a leather belt around his waist, and he ate locusts and wild honey. 1:7 He proclaimed, “One more powerful than I am is coming after me; I am not worthy to bend down and untie the strap of his sandals. 1:8 I baptize you with water, but he will baptize you with the Holy Spirit.”
1:9 Now in those days Jesus came from Nazareth in Galilee and was baptized by John in the Jordan River. 1:10 And just as Jesus was coming up out of the water, he saw the heavens splitting apart and the Spirit descending on him like a dove. 1:11 And a voice came from heaven: “You are my one dear Son; in you I take great delight.” 1:12 The Spirit immediately drove him into the wilderness. 1:13 He was in the wilderness forty days, enduring temptations from Satan. He was with wild animals, and angels were ministering to his needs.
What can we learn from this comparison? As spelled out in the subchapter before, Mark skips this poetic, wonder and mystery laden beginning of Marcion’s Gospel, although he retains the specific term, The Gospel (eujaggevlion). To remove, however, from this title Marcion’s connotation of an angelic message of an angel-like Christ who had been unknown before by any of the Old Testament prophets – he contrasts The Gospel straight after the headline with precisely a mixture of prophetic statements. These statements directly contradict Marcion’s claim that the prophets did not know of Jesus Christ, an element which we can also find in Matthew’s parallel quotation of Isa. 40:3 (‘A voice cries out,“In the wilderness clear a way for the Lord; construct in the desert a road for our God”’): ‘The voice of one shouting in the wilderness, “Prepare the way for the Lord, make his paths straight”’ (Matth. 3:3). But different from Matthew who has removed the Marcionite term ‘Gospel’ (eujaggevlion) entirely from his book opening, hence, also has no need to correct its interpretation, Mark who, indeed, does use this term gives it a special explanation:
1:1 The beginning of the gospel (= delightful message) of Jesus Christ, the Son of God. 1:2 As it is written in Isaiah the prophet, ‘Look, I am sending my angel ahead of you, who will prepare your way (Ex. 23:20; Mal. 3:1), 1:3 the voice of one shouting in the wilderness, ‘Prepare the way for the Lord, make his paths straight’ (Mark 1:1-2).
Jesus Christ is qualified to be the ‘Son of God’, and the ‘beginning of the gospel’ is not a delightful message of Jesus Christ, but about him, as the next verse highlights. The prophet Isaiah knew already about Christ and he announced already that prior to the coming of Christ, a messanger would be send by God to ‘prepare’ his way, a ‘voice’ to announce the message about the future Christ. The ‘gospel’ has been re-termed from being a wonderful wonder by a wonderful messanger, an angelic Christ, to becoming a prophetically announced delightful message of John the Baptist about the arrival of Christ. A dependency of Marcion on Mark could illustrate why he could have used the term ‘Gospel’ for his own work (something he could not have found in Luke), but then, this would not explain, why authors up until Irenaeus were so sceptical in adopting this terminology for the words and deeds of Jesus Christ, but equally why Marcion would have used it at all, having read the specific prophetic notion in Mark. In contrast, Mark’s explicit combination of Old Testament references give an anti-Marcionite interpretation to the term ‘Gospel’. With the very first part of that mixed quote from, Exodus, Malachi and Isaiah, the author even connotes ideas that Marcion voiced, but did not relate to the Old Testament prophets, whereas it is Mark’s strategy – very similar to that deployed by Justin, Irenaeus and Tertullian – to counter-argue Marcion that those elements were already foretold by the prophets. In this case it is Malachi who predicted:
“I am about to send my messanger, who will clear the way before me. Indeed, the Lord you are seeking will suddenly come to his temple.
The sudden arrival of the Lord that Marcion insisted on is nothing new, but already Malache had foreseen it, announced it and predicted his forerunner, who Mark identified with John the Baptist on which he carries on in his first chapter – a long passage that, of course, has no equivalent in Marcion’s Gospel.
From this part alone, we cannot decide about the direction of who borrowed from whom. It is, however, enlightening if we read Mark’s immediate beginning of his text against Marcion’s background:
1:1 The beginning of the gospel of Jesus Christ, the Son of God. 1:2 As it is written in Isaiah the prophet, “Look, I am sending my messenger ahead of you, who will prepare your way, 1:3 the voice of one shouting in the wilderness, ‘Prepare the way for the Lord, make his paths straight.’”
Why would Mark begin to introduce his book that is going to be a narrative on Jesus’ life, deeds and words not with a message of Christ, but with John the Baptist as a messanger with a message about Jesus Christ? Of course, Christian readers who are so used to the Synoptics’ accounts of John the Baptist may no longer feel this strange character of a delayed opening. In contrast, if we ask the question, could Mark have added this passage to counter one of Marcion’s challenges, the answer is readily at hand. As shown in the previous paragraph, The Gospel incorporated a passage where it dealt with John the Baptist’s attitude to Jesus Christ. Very similar to the other disciples, the twelve and the 70 – he was curious and enquired who Jesus was, but despite all the deeds of Jesus, he did not trust and believe, but took offensive at Christ. Mark disputes this characterisation of the Baptist, by portraying him as the messanger and fore-runner of Jesus who not only prepares his ways and baptizes him, but who also captures all the typically Marcionite features: asceticism, the emphasis on repentance, baptism and the remission of sins, but without the renounciation of the Jewish past!
Hence, he combines Marcionite and anti-Marcionite elements in his own opening:
1) The title of Marcion’s book on Jesus words and deeds, The Gospel, was adopted, but adapted with a shift of meaning from an angelic and wonderful message of the angelic Christ in Marcion to a message of a prophetic messanger who announces Jesus Christ in Mark.
2) Marcion’s criticism of the Jewish prophets and the serverance between Judaism and the Christian movement have been taken seriously, but counter-acted by the introduction of references to the Torah and the Prophets (Exodus, Malachi, Isaiah).
3) Marcion’s criticism of John the Baptist who was not more apprehensive than any of the other disciples (with the exception of Paul, of course!), and took offense at Christ, is introduced as the prophetic messanger of Christ.
4) John the Baptist himself is portrayed as a counter-Marcion (ascetic, preacher of repentance, baptism, remission of sins) who, however, builds the link between past, present and future.
5) That Mark’s anti-Marcion reading of The Gospel’s pericope on John the Baptist (Luke 7:18-23) was still present when John wrote, can be seen from the connection that John 1:19-23 makes between Mark 1:1-4 and this passage of The Gospel.
 In: J. Schäfers, Erklärung (1917), 4-5 (own trans.); S. Moll, The Arch-Heretic (2010), 119-20 has added good reasons that this commentary refers to Marcion’s Gospel.