The more I look at the opening chapter of Mark, the more I become dissatisfied with its reading. On the simple theory of Markan priority, we would have to conclude that Mark mentioned a few things without detail and that later writers expanded upon what Mark had written. These being namely the arrest of John and the temptation by Satan.
In Mark these things are mentioned very briefly, with little purpose and no background. It seems to me that such statements would have been confusing to a reader who had no other knowledge of the Gospel story. If the Markan narrative were the very first narrative like this about Jesus to be written, wouldn't the writer want to provide more detail so that the reader would understand what was being talked about?
In Luke, however, all of these things are better explained and would make more sense to a first time reader.
2 just as it is written in Isaiah the prophet:
“Behold, I am sending My messenger before You, Who will prepare Your way;
3 The voice of one calling out in the wilderness,
‘Prepare the way of the Lord, Make His paths straight!’”
4 John the Baptist appeared in the wilderness, preaching a baptism of repentance for the forgiveness of sins. 5 And all the country of Judea was going out to him, and all the people of Jerusalem; and they were being baptized by him in the Jordan River, confessing their sins. 6 John was clothed with camel’s hair and wore a leather belt around his waist, and his diet was locusts and wild honey. 7 And he was preaching, saying, “After me One is coming who is mightier than I, and I am not fit to bend down and untie the straps of His sandals. 8 I baptized you with water; but He will baptize you with the Holy Spirit.”
9 In those days Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10 And immediately coming up out of the water, He saw the heavens opening, and the Spirit, like a dove, descending upon Him; 11 and a voice came from the heavens: “You are My beloved Son; in You I am well pleased.”
12 And immediately the Spirit brought Him out into the wilderness. 13 And He was in the wilderness for forty days, being tempted by Satan; and He was with the wild animals, and the angels were serving Him.
14 Now after John was taken into custody, Jesus came into Galilee, preaching the gospel of God, 15 and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.”
16 As He was going along the Sea of Galilee, He saw Simon and Andrew, the brother of Simon, casting a net in the sea; for they were fishermen. 17 And Jesus said to them, “Follow Me, and I will have you become fishers of people.” 18 Immediately they left their nets and followed Him. 19 And going on a little farther, He saw James the son of Zebedee, and his brother John, who were also in the boat mending the nets. 20 Immediately He called them; and they left their father Zebedee in the boat with the hired men, and went away to follow Him.
Firstly, note that v14 simply says, "after John was taken into custody." What what? When was John taken into custody? Why? This sounds like something written by someone who knows that the story of John's arrest is already well known to the reader. Secondly, what's up with the very brief mention of Jesus being tempted by Satan? Again, this sounds like something written by someone who knows that this narrative is already known and doesn't want to waste time on it. Satan really plays no other role in the story or Mark either. There is no clear connection made between the demons Jesus deals with and Satan.
Now Luke:
“The voice of one calling out in the wilderness,
‘Prepare the way of the Lord,
Make His paths straight!
5 Every ravine will be filled,
And every mountain and hill will be lowered;
The crooked will become straight,
And the rough roads smooth;
6 And all flesh will see the salvation of God!’”
7 So he was saying to the crowds who were going out to be baptized by him, “You offspring of vipers, who warned you to flee from the wrath to come? 8 Therefore produce fruits that are consistent with repentance, and do not start saying to yourselves, ‘We have Abraham as our father,’ for I say to you that from these stones God is able to raise up children for Abraham. 9 But indeed the axe is already being laid at the root of the trees; so every tree that does not bear good fruit is cut down and thrown into the fire.”
10 And the crowds were questioning him, saying, “Then what are we to do?” 11 And he would answer and say to them, “The one who has two tunics is to share with the one who has none; and the one who has food is to do likewise.” 12 Now even tax collectors came to be baptized, and they said to him, “Teacher, what are we to do?” 13 And he said to them, “Collect no more than what you have been ordered to.” 14 And soldiers also were questioning him, saying, “What are we to do, we as well?” And he said to them, “Do not extort money from anyone, nor harass anyone, and be content with your wages.”
15 Now while the people were in a state of expectation and they all were thinking carefully in their hearts about John, whether he himself perhaps was the Christ, 16 John responded to them all, saying, “As for me, I baptize you with water; but He is coming who is mightier than I, and I am not fit to untie the straps of His sandals; He will baptize you with the Holy Spirit and fire. 17 His winnowing fork is in His hand to thoroughly clear His threshing floor, and to gather the wheat into His barn; but He will burn up the chaff with unquenchable fire.”
18 So with many other exhortations he preached the gospel to the people. 19 But when Herod the tetrarch was reprimanded by him regarding Herodias, his brother’s wife, and regarding all the evil things which Herod had done, 20 Herod also added this to them all: he locked John up in prison.
#1 The quote from Isaiah is cleaner in Luke.
#2 Here the writer explains why John is arrested up front. The story makes more sense.
5 And he led Him up and showed Him all the kingdoms of the world in a moment of time. 6 And the devil said to Him, “I will give You all this domain and its glory, for it has been handed over to me, and I give it to whomever I want. 7 Therefore if You worship before me, it shall all be Yours.” 8 Jesus replied to him, “It is written: ‘You shall worship the Lord your God and serve Him only.’”
9 And he brought Him into Jerusalem and had Him stand on the pinnacle of the temple, and said to Him, “If You are the Son of God, throw Yourself down from here; 10 for it is written:
‘He will give His angels orders concerning You, to protect You,’
11 and,
‘On their hands they will lift You up,
So that You do not strike Your foot against a stone.’”
12 And Jesus answered and said to him, “It has been stated, ‘You shall not put the Lord your God to the test.’”
13 And so when the devil had finished every temptation, he left Him until an opportune time.
Here the temptation of Jesus has a purpose. This demonstrates that Satan/the devil, is the "lord of this world". V6 plainly indicates that the world is ruled by Satan/the devil, just as the sect at Qumran and other Jews believed. But this point is never made in Mark, who skips the details that make the scene relevant.
24 When the messengers of John had left, He began to speak to the crowds about John: “What did you go out into the wilderness to see? A reed shaken by the wind? 25 But what did you go out to see? A man dressed in soft clothing? Those who are splendidly clothed and live in luxury are found in royal palaces! 26 But what did you go out to see? A prophet? Yes, I tell you, and one who is more than a prophet. 27 This is the one about whom it is written:
‘Behold, I am sending My messenger ahead of You,
Who will prepare Your way before You.’
28 I say to you, among those born of women there is no one greater than John; yet the one who is least in the kingdom of God is greater than he.” 29 When all the people and the tax collectors heard this, they acknowledged God’s justice, having been baptized with the baptism of John. 30 But the Pharisees and the lawyers rejected God’s purpose for themselves, not having been baptized by John.
The comment about the clothing is interesting. The statement implies that John did not meet the expectations of the people who sought him. They were looking for a prophet, but expected him to be more presentable. They expected him to be well dressed.
Interestingly, Mark describes how John was dressed, and uses a passage from the scriptures to indicate that John was dressed like a prophet.
I've said that I have a problem with the idea that one could take a story written without scriptural references, and then be able to find scriptural references to write the same story around. But I think this is an example where that seems plausible.
Yet at the same time, I have a hard time believing that this could be true of all of Mark. What I do know is that much of Mark 1 seems to me like it was written by someone who had knowledge of narrative elements that are found in Luke and knew that their readers would be aware of those details. (Also note that some of those elements are not found in Marcion.)
I don't have an answer here. I also don't think that any conclusion can be made about the entirety of any work. I think its possible that parts of Mark were written before parts of Luke and parts of Luke were written before parts of Mark. What I do know is that to me the opening of Mark reads more like a summary of material found in Luke than the material in Luke looks like an expansion upon what is found in Mark.