The only place Jesus' baptism is mentioned is in the Gospels, starting here with what I just quoted. You can't argue that because baptisms were common that therefore some common baptism must lay at the root of this obviously mythological one. People have killed lots of lions before; that doesn't mean there's some historical kernel of truth laying behind the myth of Hercules killing the Nemean one.
Well, the clear majority of scholars are not denying that Christian sources are the basis for Jesus's
particular baptism by John, so I view this as a red herring that doesn't actually help validate or invalidate the claim (as its not even up for debate!). Practically everything we know about Jesus comes from Christian sources. The whole point of the scholarship is to sift through these sources using a rigorous academic methodology to determine which 'layers' represent the most primitive and thus 'better-candidates for historicity' elements of the tradition and which should be regarded as later theological accretions.
Your allusion to Hercules killing the Nemean lion is not a convincing one, because no scholar of antiquity accepts that Hercules actually existed whereas there is near unanimity amongst relevant experts that Jesus did. Nor are there any contemporary historical sources evidencing that there
were 'lions' like the Nemean one described in the myth being slain by people in a manner akin to that of Hercules, such that we could deduce some plausible historical core to the myth. But we do have such parallels in our 'baptism' case from the only extant historical account of that period to have remained from antiquity, Josephus.
That we have independent historical testimony from Josephus that John the Baptist had a ministry of baptism of fellow Jews in this region at this time, is important in assesing the plausibility of the New Testament accounts (including the possibility that Jesus was a subject of such a baptism ritual). Were we lacking the former, then we would have reason to doubt the entire schema of a baptising itinerant preacher whom Jesus encounters and becomes affiliated with before starting his own ministry. But the independent historical testimony provides us with an important link in that chain. It is not decisive in proving the historicity of the latter but it does make it 'plausible' (which, when combined with other criterions, convinces most biblical scholars that it is also
probable).
It certainly makes it far more plausible than your allusion to the Nemean lion.
Mark, Q (according to a number of scholarly assesments that it appears to have contained a different version of the baptismal pericope that was used by Matthew/Luke over against Mark's account) and John all portray Jesus as having been either baptised by John and/or his own ministry beginning after John's ministry of baptism.
There is nothing implausible in this portrait.
As some scholars have argued, the criterion of embarassment is a bit anachronistic. What would be embarrassing to us is not necessarily embarrassing to people in another culture. One of the key themes of Jewish and Christian spirituality is that one must be humbled to be exalted. The first shall be last and the last shall be first. The foolish things of this world have been chosen over the wise. Etc. Jesus humbling himself to be baptized fits perfectly with this central Christian theme.
There certainly are critics of the 'criteria of authority' approach, such as Dale Allison, yes (even though it is the gold standard in scholarship) as I demonstrated earlier in my discussion with firedragon. But Allison too doesn't deny the historical probability of Jesus having been baptised by John under his 'pattern' approach (which is concerned with 'general impressions' as opposed to the historical/unhistorical likelihood of individual pericopes). So the allusion to those 'some' scholars does not actually get around the fact that there is a consensus here amongst the ranks of the qualified experts in this field.
In his
Constructing Jesus, Allison notes:
"Indeed, Jesus seems to have submitted to John’s baptism. . . .This is rarely doubted" (p. 53)
Now, I'll grant that you may have a point in suggesting that 'humility' is a cardinal virtue in much early Christian literature. But the 'submission' to John's baptism is far more theologically problematic than depicting Jesus as humbly 'serving' the needs of others rather than being served himself but still from a position of
exaltation as God's chosen, because it contradicts other 'patterns' that are fundamental to Mark's theology.
Mark does not pay any attention to the '
kenotic' theology we find in Paul's letters - where Jesus is a pre-existent divine being who humbles himself to take on human form and is then exalted after his crucifixion. He has an exaltation theology, in which Jesus is the prophesised 'chosen one' - predestined by God for election as his agent of eschatology and divine redemption, indeed the 'Son of Man' coming with the clouds of heaven (by allusion to Daniel in Mark's passion narrative). The narrative continually hits home that there is no one to compare with Jesus in authority - “Son of God” as a christological title occurs in Mark 3:11 and 15:39, the Lord of the Sabbath (2:28), has the authority to forgive sins (2:10), sits on the right hand of the Mighty One (14:62) and with great glory will judge the entire world flanked by his angels (8:38; 13:26).
And nothing about that specifically Markan account appears to match up with Jesus submitting himself to the baptismal authority - that is the religious authority, power and status - of
another figure claiming to be divinely inspired, another human personage with an eschatological mission who isn't portrayed as God's Son, the Mighty One sitting on a throne and coming with the clouds of heaven or anything remotely akin to this.
That is not to refute your accurate reflection as to the importance of humility and indeed 'passive suffering' (in lieu of Mark's individuated exegesis and Messianic application of Isaiah's suffering servant in chapter 53 to Jesus) but I'm not convinced by this as a plausible reason for why Mark would include a scene showing Jesus being 'under' the authority of another figure who could have (in the early years of the tradition) potentially competed with him for status.
Mark himself and the later gospel writers appear to have been unsettled by the implications, which is why the 'validation' from God involved with the Spirit coming down becomes more grand with each telling - until John doesn't even describe the actual scene of the baptism in his gospel but has the Baptist orally declare that he saw the "dove" descend from on high and alight on Jesus as the only Son of God.
Again, I can't help but think that if this were any other myth, you wouldn't be putting as much effort into trying to somehow squeeze some plausible thread out of this obviously mythological story.
If I may ask, what is 'obviously' mythical about a person in first-century Judea being baptised by another person whom we know on good exterior authority actually baptised many Jews in that region, speaking about eschatology (like the person who baptised him) and after that figure's execution - for fear of inciting rebellion - taking over a mantle of leadership with some of his disciples and proceeding to initiate messianic fervour himself that ends up with this person being crucified (attested also in Tacitus and in the uninterpolated part of Josephus that most scholars concur with, as well as Paul's letters dating from the 50s CE), something the Romans did to upteen other people for crimes of slave rebellion, brigandry or sedition in this pre-70 CE time period?
I see no good reason for either the 'baptism' or the 'crucifixion' being contrived, as no Jews of this period were expecting a baptised and crucified Messiah - and moreover why would the Christians invent the tragic execution of their Jewish leader to try and convince Jews that he was the messiah, or indeed Romans that someone dying the most shameful slave-death on a Roman
stavros as a dissident of the Empire might be a good person for them to follow?
It just doesn't fit with the Jewish 'prophecies' they are alleging that Jesus fulfilled or something that would seem logical for them to have contrived as a myth in that cultural context for the audiences in question.
Rather, the most plausible deduction is that this figure actually was both baptised and crucified - and after the fact they had to try and makes these unfortunate incidents "fit" with the theological agenda they desired.
Which is what the consensus of the scholars actually is.