One of the things that many scholars note is that the evangelists seem to have been embarrassed by Jesus' baptism by John. It was such common knowledge when Mark wrote his gospel c. 70 CE, so they say, that Mark's gospel had built in apologetics for it (cf Mk 1.7 “one whose sandals I am not worthy to stoop down and untie“) and later evangelists were even more embarrassed by it than Mark (cf Mt 3.14).
What is odd to me about this entire line of reasoning: Who knew that Jesus was baptized? In Mark, Jesus is an absolute nobody until well after his baptism; no one knows who he is when he starts preaching in the synagogue at Capernaum (Mk 1.27-28), and he doesn't call his first disciples until after he spends forty days in alone the desert; forty days after his baptism. And only after John is put in prison does Jesus start preaching at all or call his first disciples. So how did the first disciples know that Jesus was baptized? Where they there at the Jordan? No one in any of the narratives in any of the canonical Synoptic gospels seems to show any awareness of Jesus having been baptized. So in order to claim that Jesus' baptism was so well known, one would have to simply assert such a fact since such knowledge is never expressed in the Synoptic gospels.
At John 3.22 / 4.1 Jesus baptizes but then the author corrects himself by saying that it was the disciples baptizing and not Jesus. But most scholars claim that John's gospel is useless as far as reconstructing early Christian history.
Stranger still, no one in the Synoptic gospels are ever baptized. Was Peter baptized? John and James? Matthew? Levi? It is never stated that Jesus baptized any of his disciples, and the only time that “Jesus” baptizes – or charges his disciples with baptizing new Christians – is in the very last line of the resurrection narrative in Matthew and in the added ending in Mark (which was probably added after John was written).
It seems like baptism was already something that Christians had been doing as part of their initiation ritual, and Mark wrote a story about Jesus being baptized to function as sort of a typology for all Christians. Jesus began his ministry at his baptism, confessing his sins, this scene represents something that all Christians do at the beginning of their new identity as a Christian. Therefore this baptism of Jesus is not “historical” in the sense that it is something that a historical Jesus actually did. It is a scene invented by Mark as a sort of etiology for Christian baptism.