Thought to be the earliest of the canonical Gospels, Mark is also the shortest. I can best explain Mark in comparison to Matthew.
Much of Mark is identical to Matthew, so scholars agree some of these guys copied from each other. Most think Matthew and Luke copied from Mark. There are differences too, some of them remarkable.
How Mark Is Different
Mark skips the genealogy and early stories of Jesus and his parents. It dives right in. John the Baptist is already preaching in the first few verses. And by 1:9, adult Jesus shows up “from Nazareth in Galilee” ready to be baptized and tempted by “Satan” (it was “the devil” in Matthew). Before the first chapter ends, Jesus has already called his first disciples, cast out “impure spirits”, “healed many who had various diseases”, “drove out many demons”, cured leprosy, and more.
Matthew attempted (but failed) to present Jesus as the long-awaited Messiah who performed miracles by the power of God. But Mark just presents Jesus as an unhinged magical wizard or sorcerer who gives many confusing speeches and unexplained parables.
His own family said “he is out of his mind” (3:21) — and a few verses later he disowned them (3:32-34). One woman was healed by touching Jesus’ clothes (5:28-29) and Jesus “realized that power had gone out from him” and interrogated the crowd “to see who had done it” (5:30-32). He used magic words (“Talitha koum!”) to raise a girl from the dead (5:41), stuck his fingers in a man’s ears to cure deafness and touched a tongue to cure muteness (7:33) with more magic words (“Ephphatha!”, 7:34), and spit in a man’s eyes to cure blindness (8:22-25).
Unlike other Gospels, Mark does not report that Jesus was seen after his resurrection. (The final 12 verses, 16:9-20, are known to have been attached later and do not belong in the original manuscript.)
For no reason at all, Mark reports a naked man fleeing the scene after Jesus was arrested (14:51-52). Go ahead and explain to me why an omniscient God put those two verses in the Bible. Or perhaps Mark, like many Christian preachers today, was just thinking about naked men.
Even in the long stretches of text that seem identical to Matthew, some minor details are changed.
How Mark Is The Same
Some of the weirder parts are repeated. Mark repeats Matthew’s claim that Jesus said he would be dead for three full days, despite it only being one day and two nights. Mark agrees with Matthew that Jesus claimed the End Times would occur during the lifetime of the current generation (9:1; 13:30). He repeats the claim that rich people don’t go to heaven (10:25).
Like Matthew, Mark reports on events that no one could have witnessed — because Jesus was alone.
Mark disagrees with Matthew on the location of the pig-demon-massacre — and Mark’s location is 30 miles from the sea that the pigs supposedly ran into — “down the steep bank”. (And Matthew has two demon-possessed men in this scene, while Mark only has one.)
Mark names three women who went to Jesus’ tomb “after sunrise”, which is different than Matthew’s two women. Mark says “a young man” awaited the women at the tomb, while Matthew reports “the angel of the Lord”. The speech by this young man is different than the speech reported in Matthew. Mark reports that the women “said nothing to anyone because they were afraid”, while Matthew says the women ran to tell the disciples.
After multiple descriptions of Jesus raising people from the dead, Jesus’ disciples seem not to have heard of it. When Jesus told them he would die and “rise from the dead”, “they kept the matter to themselves, discussing what ‘rising from the dead’ meant” (9:9-10). If they’d already seen this happen, why would they wonder what it was?
Taken alone, Mark is less believable than Matthew. The writing is less careful and loosely organized. The writer focuses more on mystery and magic rituals.
But taken in comparison to Matthew, there are needless contradictions — which detract further from the credibility of either Gospel.