pakeha
Other than his epistles, all the mention we have of Paul is in Acts?
There are many non-canonical early mentions of Paul as well. Some things are serious, like
1 Clement (a letter which is addressed to the Corinthian church, and draws on Paul's traditional authority as founder there), while other things are frankly fictional like
Acts of Paul and Thecla (fun to read, too). There is Gnostic material with an alternative Paul. There are fake (umm, pseudepigraphical) epistles of his, besides the half-dozen in the canon.
But I don't know of anything that's early, besides what you mention, about Paul's pre-conversion activities. Maybe there is, and I just don't know it. The absence is a little strange, since so many Christians are so heavily invested in their being persecution victims (even today), and the religion itself is so much about being a sinner and then repenting to find redemption.
The evolution of a John the Baptist connection
Baptism was practiced by Paul, maybe reluctantly, and he discusses it with a hint that some people keep track of who baptized whom (
1 Corinthians 1: 13-17). Paul doesn't mention John at all in any survivng letter.
We don't know why
Mark was written, and its sole purpose needn't have been to supply a biography of Jesus. Part of the purpose may have been to explain church practices, as those were coming to be regularized by an institutional church.
If we look at Josephus' historical Baptizer (
Antiquities 18, 5, 2) the wet ritual itself does not confer forgiveness, but is part of a larger episode of achieving righteousness,
... the washing would be acceptable to (God), if they made use of it, not in order to the putting away of some sins only, but for the purification of the body; supposing still that the soul was thoroughly purified beforehand by righteousness.
A baptism by Josephus' John would be no occasion of "embarrassment" even for a Godman, but rather a proclamation of thoroughly purified righteousness. Woohoo. But what's in received
Mark 1: 4-5?
John the Baptist appeared in the desert proclaiming a baptism of repentance for the forgiveness of sins. People of the whole Judean countryside and all the inhabitants of Jerusalem were going out to him and were being baptized by him in the Jordan River as they acknowledged their sins.
There is confession, a Christian practice, a blurring of whether the baptism itself or the sinner's repentance accomplishes the forgiveness, and not one word about the soul being thoroughly purified
beforehand by righteousness. We also have some acknowledgment that the Christian practice is different from John's, at 1:9
I have baptized you with water; he will baptize you with the holy Spirit.
I think there is some connection between the earliest Jesus movement and the Baptizer's historically attested activity. Maybe Jesus actually did some work with John, and got dunked. Jesus' baptism experience, at least in
Mark, has a psychological ring of truth. Don't eat or sleep for a few days while some holy wildman manipulates your mind set. That's the thorough purification beforehand. On the big day, do some calisthenics in the desert sun, then jump into cold water and look up at the sky. There's no telling what you'll see and hear.
Then what? There's no intellectual property laws to worry about. Anybody could "preach John," whether John agreed or not. So, I doubt we could ever recover Jesus' having been a formal disciple of John, even if Jesus was. Lots of people admired John, why not Jesus and the James Gang? Baptism is good theater, and a great way to meet women. Use it.
There needn't have been any embarrassment in the linkage of Jesus with John through baptism, but the passage seems overloaded with disparate intentions: explain the church's later practice, get a prestigious Jew's endorsement of Jesus' Messiahship (sort of), put the fundamentally unJewish Godman idea into play straight from the Father himself, something about a bird ... too much stuff retrojected onto too small an incident for any kind of coherence.