That said, let's jump in.
Despite what the new round of mythicists like Robert M. Price and Richard Carrier say, I can't accept mythicism, though, per Wikipedia, I do support those like R. Joseph Hoffmann who say it deserves academic discussion. Bart Ehrman, insightful as he may be otherwise, is simply wrong here.
The earthly Jesus of Paul
On Price, I believe he's simply wrong when he says Paul didn't believe in an earthly Jesus. The "born of a woman" in Galatians 4;4 flatly contradicts him and others.
For example, Earl Doherty engages in verbal flim-flammery here, to try to talk around the passage. Some of it is just laughable, like claiming, based on the Old Testament, that "exapostello" can only be used for sending forth "spiritual beings." I don't know that Price has even tried to do that much ersatz heavy lifting.
What about James?
And, the mythicists also, by and large, don't seem to address Josephus' discussion of lynching of James the brother of Jesus. James (Ya'akov) was a hugely common Jewish name, of course, but so was Jesus (Yeshua). So, Josephus seems to recognize that this James needed to be identified. He does so by calling him the brother of Jesus. But, that presumes that this Jesus is so well-known as to not need identifying. (Note; I do not see this as a Christian interpolation, certainly not the "brother of Jesus"; I am still undecided on the "who was called Christ.")
Carrier does address this, saying this was the brother of a Jesus who was high priest in the year 63. But, to follow Carrier, you have to hold the "who was called Christ" as an interpolation. You also have to assume an audience 30 years later was familiar enough with THAT Jesus for Josephus to not need to identify him further, and I'm not ready to commit to that. Per Wikipedia, Josephus' audience was Gentiles; unless you narrowly restrict that audience to "friends of the Flavian imperial dynasty," they probably wouldn't know who this Jesus was.
(And, on the personal side, that Carrier, or a minion, can't approve a comment or two on his blog 12 hours after posting it makes me look a bit askance at him anyway.)
So, I stand by some sort of non-mythical Jesus. And, I'll deal with Josephus myself, below.
Early Christian growth rates: One view
This leads to Christianist sociologist Rodney Stark. (I use that moniker because, like Samuel Huntington, he believes in a "clash of civilizations," but he's not a religious Christian, though he is a nutter in other ways.)
Stark postulated that, with a start point of 5,000 believers in the year 50 CE, and a growth rate of 40 percent per decade, we get Christians up to being half the Roman Empire by the time of the Council of Nicea.
I think that end number is too high, and I'm far from alone. I think Christians at that time were higher than a Robin Lane Fox thinks (more below) but not that high.
Anyway, how did Stark get there?
His 40 percent per decade comes from historic growth rates of Moonies and Mormons. However, there's several problems. Moonies moved their base from small South Korea to the US, and the Mormons recruited immigrants, especially in Scandinavia, before they emigrated, for one thing. Also, he fails to allow for Christians outside the empire, who may have been as high as 15 percent of the total by the time of Constantine. And, he doesn't try to fit this growth into a bell curve, especially given that total population in the ancient world remained almost flat, and indeed did so, over longer periods, until about 1700 CE.
Now: My alternative
So, what if we "move Jesus back"?
Why not? G.R.S. Mead, and others, either entirely on their own, or influenced by the Jesus Pandera tales from pre-Rabbinic Judaism, postulate a Jesus who lived circa 100 BCE, possibly a Jesus who was among the Pharisees executed by Hasmonean king Alexander Jannai. (Here's Mead on this.)
This idea has long appealed to me. Mead and former mythicist G.A. Wells (I consider him to be a recanted mythicist, not a "soft" one) were two major influences on me in my late 20s, beyond modern historical criticism, as I did intellectual judo on what I was being taught in conservative Lutheran divinity school as well as what I had been taught to avoid.
The idea still appeals today, maybe even more so, and for two main reasons.
We get a slower, easier-to-believe growth rate, and a longer period for development of Christian traditions and their move from oral to written forms.
First, my take on the growth rate issue
I see two "hinge points" in the growth of the Jesus movement, later Christianity. (Dating Acts to 115 CE or so, it wasn't "Christianity" before 100 CE.)
The first one is for there to be at least 5,000 Jesus followers by 50 CE. Even with allowing for uneven distribution, the number of Jews in Rome, and the religious inquisitiveness within Rome (the city, not the empire), I think this is the minimum number for a "Jesus community" to be in place for Paul, never having been there, to address in his Epistle to the Romans.
The second? Per the likes of Nassim Nicholas Taleb and other professional and amateur social theorists, is a "tipping point" that led Diocletian to start serious persecution of Christians. (Per my review of Candida Moss' "The Myth of Persecution," I don't think there was serious, imperially-directed persecution of Christians before Diocletian, but there certainly was then.)
Such sociological tipping points are very serious. Urban sociologists have shown that when an all-white, or nearly so, neighborhood, gets X percent African-Americans, the rate of moving out picks up. When it hits Y percent, the rate increases.
I see a similar "tipping point" behind Diocletian. What might it be?
I started playing around on a simple computer based 10-key calculator. I started with a much earlier time frame, which allowed lower growth rates over a longer period, but also gradually accelerated growth rates as Christianity got larger mass.
And, I came out with approximately 1.25 million Christians in the year 285. If I assume an imperial population of 50 million, and act like Stark and assume all these Christians were inside imperial boundaries, that's 2.5 percent. Given Christianity's strength in the east, maybe 3 percent or a little bit more, counting Italy and Roman Africa (today's Tunisia) as part of the east.
And there you go.
I assume 1,000 "Christians" in the year 70 BCE.
From there, I assume a growth rate of 10 percent per decade until 30 BCE. (Again, I'm assuming flat population rates, in part for simplicity but in larger part because, over the longer term, that's the simple reality of the ancient world.)
That gives us about 1,460 "Christians" in 30 BCE.
For the next six decades, I up the growth rate to 15 percent, starting a bell curve. (The proclivities of Herod, followed by direct Roman control of Judea, are assumed as "stimulators" for this higher growth rate.)
At 30 CE, that gets us to about 3,380 "Christians."
I then ratchet the bell curve up a bit, to 20 percent growth over the next 70 years. Worsening Roman action in Judea and the first revolt, plus the work of a man named Paul, are taken as "stimulators."
That gets us to 12,130 "Christians" (per above on the book of Acts, the scare quotes will be dropped from here on out) at 100 CE.
I next ramp up the bell curve a bit more.
For the next 80 years, I assume 25 percent growth per decade. This gets us to about 72,300 Christians by the death of Marcus Aurelius. Still a tiny minority, not much more than 1/10 of 1 percent of the Empire. The myth of Paul before Felix and Festus aside, we now, with people like Justin Martyr, see the first interactions of Christians with Roman officialdom. Somebody is noticing them. That's a "stimulator" of growth.
But now, the growth rate is going to pick up again. No external stimulator, just a larger semi-critical mass, more publicity in non-Christian circles, and social and economic decay in Rome making some people look more favorably on Christianity. (The movement, also, now has no scare quotes, as first attempts at doctrinal organization begin.)
So, for the next 90 years, I put the growth rate at 30 percent.
That gets us to 453,000 by 250 CE, which would be 3/4 of a percent or more. And 767,000 by 270 CE, comfortably above 1 percent.
So, I turn the bell curve higher. To 35 percent.
That gets us to 1.89 million by 300 (and about 1.25 million in 285, the year after Diocletian assumed the throne).
If I keep that same growth rate for 25 more years, we're at around 4 million by the time of Constantine calling the Council of Nicaea. That's 8 percent of a population of 50 million. (And on tipping points, for Constantine, we're at around 2.75 million Christians, or 5.5 percent of the population, by the time of the Battle of the Milvian Bridge.)
I don't ever need to go to Stark's 40 percent. And, I don't even see that happening. Due to "backsliding" and the "leisure" of Christianity being legalized, the growth rate probably plateaued until Theodosius made it the state religion of Rome.
Now, the development of Christian tradition
Different ideas of Jesus, such as a teacher of wisdom, allegedly reflected by the hypothetical Q document and the Gospel of Thomas, a faith healer/wonder worker with parallels in some Jewish holy men of the era, and a "divine man" of some metaphysical import with possible soteriological import, would all have longer to ferment, and more under any communal "radar screens," with an extra century of development time, and with a slower growth rate to bring less attention to any perceived need for uniformity.
We can postulate brief written bits being put down by the second generation after the death of this Jesus, around 30 BCE, when I see the growth rate tick up a bit, or maybe after 30 CE, when I see the growth rate tick up a bit more.
We can then simply put the first Jewish Revolt against Rome, and the destruction of the Second Temple, on top of this as a motivator to record the first unified "gospel" of Jesus in light of similar gospels of wonder-workers both before and after this time that floated around the Mediterranean world.
More on and from Paul
That still leaves connecting Paul to all of this.
He claims to be a Pharisee himself in Philippians. I accept this as true. I reject that he was a Roman citizen, as he never claims that about himself. He could well have come across the "Jesus movement" as part of a trip to Jerusalem or something.
If his "thorn in the flesh" was temporal lobe epilepsy, or whatever led him to his visions and revelations, his version of Christianity took off from there. And, if he did die in Rome, or nearby, we can perhaps peg the first Gospel, Mark, to there, in part based on the "Latinisms" in its Greek. But, that's aside from the main focus of this argument.
That said, now I, in a different way than Carrier, have a problem with Josephus' statement above. And, I also have a "problem" with the thinking of the likes of Robert Eisenman on the history of Jewish Christianity. And, on the issue of James as a historic person. (That said, Eisenman takes Acts as being WAY too historic. Example: He thinks the phrase that people of the "Way" were first called Christians in Antioch first dates to the 50s CE.)
Paul and James
Of course, Mead potentially founders on this, too. Paul talks about meeting James, and Paul in his own writings, not Luke's "Paul" in Acts. (It's been a long, long time since I read Mead; I don't know how he addressed this issue.)
But there's a way around what Paul says in Galatians 1:
Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. But other of the apostles saw I none, save James the Lord's brother. Now the things which I write unto you, behold, before God, I lie not.Note he says "the Lord," not Jesus. This is usually means the risen Christ, not the earthly Jesus. Hence, not to sound like a Catholic protecting Mary's perpetual virginity, but, there's no reason this has to be read as James being the earthly brother of an earthly Jesus.
So, let's then go back to Josephus. And, let me one-up Carrier.
Perhaps EVERYTHING after "James" is a Christian interpolation. There may have been an earlier identifier of James, rather than just his bare name, in Josephus' autograph. (I admit I'm unaware of textual variants on this.) But, maybe an early copy got edited for this interpolation. In fact, if Luke borrowed from the Antiquities, he could have made that interpolation himself.
In for a penny, in for a pound on interpolations. But, that's the only one I need to get past the issue of a historic James at the time of Josephus and a historic Jesus of nearly 150 years earlier.
World religious history
I'm going to briefly morph over to other world religions, then.
The late-Victorian England original "surge" in mythicism wasn't about Jesus, alone. As the British Raj expanded in India, the historicity of the Buddha was questioned, too.
There's both similarities and differences. The early Buddhist writings might be compared to the hypothetical early Q strand, if there was a written Q. The Buddha as a teacher of wisdom, occasionally esoteric, just like Jesus in this tradition. The difference is that the idea of the Buddha as a divinity didn't develop until later, so no textual fusions were needed. But, if we push back Jesus, we have about the same gap between his life and the first writings about him as we do the Buddha and the first writings about him.
Islam is a tougher nut, with so little textual criticism so far. But, it's possible we didn't have a finalized version of the Quran until a century after his death or so.
Finally, to tackle one other "hard mythicist" counter-thrust.
Now, I've seen one mythicist (sorry, no link, can't remember where it was) cite the Micronesian cargo cults, specifically noting that in one place, two different deities or whatever were being venerated in less than 20 years.
However, on the typical Micronesian or New Guinean site, the issues that started the cargo cult were HUGELY intrusive. A white man crashing an airplane during World War II into a dark-skinned, often Stone Age, society. And then leaving as soon as rescued.
Whereas, a messianic movement starting in late Second Temple Judaism? With all the other messianic claimant that Josephus documents in his time and others probably earlier? Hardly intrusive at all.
My academic and personal background
Who am I to be laying this scenario out with any presumption to credibility?
I'm a "semi-academic," in case any scholars noted in this post, or friends or followers of theirs, question my presumption.
I have an undergraduate degree in classical languages, part of spending most of my time in a pre-divinity program. (I also read Hebrew as part of that, though not enough to get a biblical languages degree.)
I also have a "professional/terminal" masters, a master of divinity degree. Being at a conservative Lutheran seminary, we weren't taught too much about the critical theological method, but we were taught a little. And, from there, I did a lot of reading on my own, eventually doing intellectual judo to refute what I was being taught there and had been raised to believe.
That said, I had several classes in exegesis of particular biblical books. I audited a class on introductory Aramaic. I took a class on the Greek of the Septuagint and also did readings in patristic Greek. And I took an upper-level graduate course on textual criticism. And, in English, I've read much of the Dead Sea Scrolls, most of the Nag Hammadi finds and more.
And, in the years since graduation, I have kept up with a fair amount of biblical studies on both testaments.
So, I feel comfortable calling myself a semi-academic.
And, I feel quite comfortable, as a semi-academic, and as an atheist who's not a Gnu Atheist, telling the mythicists they're wrong, and how and why.
Anyway, this isn't an overarching redaction criticism of any specific New Testament book, or some other detailed academic exercise. For example, I'm not going to try to tackle how a Mark relocated Jesus 100 years earlier. Suffice it to say that, albeit with a mythical leader, we historically had "early" and "late" dates for the Jewish exodus from Egypt, and the certainly mythical Zoroaster has had floruits dated as much as 1,000 years apart.