The first thing to say is that I haven't yet read Atwill's book, Caesar's Messiah: The Roman Conspiracy to Invent Jesus, only claims about it by others, some of which are clearly copied or based on press handouts prior to the symposium Covert Messiah at Conway Hall in Holborn, London, on October 19.
As I understand it, Atwill's 'proof' is a series of close parallels between the military campaigns of Titus Flavius (a military general under his father, the Emperor Vespasian, before succeeding his father as Emperor Titus) and the ministry of Jesus. According to Atwill, if I've understood things, the Gospels were simply made up in Rome, probably by Josephus. Apparently these parallels were coded 'confessions' which would have been readily understood by the Roman elite that he had written it so they wouldn't take it seriously. The argument is that Josephus concocted a history of Jesus loosely based on the biography of Titus in an attempt to give the troublesome Jews a Messiah to follow in accordance with their belief that a Messiah was soon to come. This one would of course tell them to be gentle, turn the other cheek, and be good, peaceful citizens of the Empire.
What seems to have eluded many scholars is that the sequence of events and locations of Jesus ministry are more or less the same as the sequence of events and locations of the military campaign of [Emperor] Titus Flavius as described by JosephusNow, attractive though that theory might be, it isn't proof on its own, let alone irrefutable proof, and it begs a lot of questions.
One thing we need to be wary of is the temptation to accept a book by it's title or by the few snippets of information we have read about it. This is a trap which so many religious people fall into, especially fundamentalists and creationists, who will gladly buy up books which tell them what they want to read and then wave them around as proof of whatever wacko superstition or bad science they are trying to push. We've all seen them embarrassing themselves using long-refuted arguments or parrotting nonsensical drivel they've read in a creationist book.
Note. I'm not saying that Joseph Atwill's book falls into this category, only that we need to examine the evidence he presents very carefully before declaring it evidence, let alone proof of anything.
Some of the questions which spring to my mind are:
If Josephus wrote the gospels then the very compelling arguments put forward by, amongst others, Dan Barker, that the so-called Testimonium Flavianum, which is often held up as proof that Jesus existed, was in fact a forgery, becomes problematic. Was it simply that Josephus forgot to mention the fictitious Jesus he had created earlier so someone had to add it later?
Then there is the evidence from the very many analytical studies of the Gospels that they grew and developed out of earlier, now lost, sources as writers adapted the Jesus myth to suit their own ends in a struggle for control of the various factions and sects which were around at the time. This would be very difficult, if not impossible, to forge.
Set against this we have tentative evidence of Roman involvement in the creation of the Jesus myth. For example, the account of Paul's conversion on the road to Damascus was clearly made up by someone who was not from the area and probably not from the time in which it was set. It was written by someone who did not realise that the writ of the Temple authorities in Jerusalem, in Judea, did not extend to Damascus, in Phoenecia, so 'Saul' could not have been on a mission to arrest Christians in Damascus and take them to Jerusalem for trial. This is clearly made up, and quite possibly in Rome. But Josephus would very likely have been well aware of the situation vis á vis the lack of Judean religious authority over Damascus, so he is unlikely to be the one who made it up.
We also have the widely assumed to be interpolated account of Jesus selecting Simon bar-Jonah to be 'Peter' the rock on which he was to build his church. The story reads like a badly thought-out tale and actually presents Jesus as a poor judge of character and a foolish man who built his house on sand. It is widely assumed, especially in Protestant circles, to have been inserted to give credence to the Pope's claim to be the spiritual descendant of Peter and thus leader of Jesus's own church. In fact we know that the post of Pope derives not from some putative first Christian bishop of Rome but from the pre-Christian Roman God-Emperor, or Pontifex Maximus, the first one of which was Augustus who combined the post of High Priest (Pontifex Maximus) and Emperor and declared hiself a living god. Christianity has clearly been grafted onto this pre-Christian Roman administrative device.
In fact, the name Peter and even Jesus, may well come from a muddle over the existing words for God the Father coming from an Indian god Dyaus combined with the Greek for 'the father', patêr, to give Zeus Patêr (father of gods), which transmuted into Jesus and Peter as well as Jupiter. In Egypt, Zeus Patêr had become Iusa Krst (Osiris), son of 'Ptah' the 'father'. So we have Iusa Krst and Ptah. Some years later, and having little knowledge of the gods and their names from the Egyptian/Judean area, Romans could well have written about Jesus and Peter. Muddle that with the Greek for 'rock', petra, and you have Jesus choosing Petra as his first disciple and successor.
To me, a compelling narrative, though I'm always prepared to revise it, is that Jesus 'the Christ' (i.e., the Jewish Messiah) is based on a possibly real, possibly mythical, member of the Nazarene sect, an apocalyptic Jewish sect founded by John the Baptist and later taken over by James 'the brother of Jesus' and John's cousin, to become the Ebionite sect. There is likely to have been several similar sects in Judea at that time as Judaism suffered the culture shock of being incorporated into the Roman Empire and becoming subject people once again, with the official religious leaders losing their authority and control as they became puppets of the Roman governors. James is very likely to have been the first bishop of Jerusalem which, in those days, simply meant the leader of a small sect, probably ultra-Jewish and apocalyptic , hence Matthew's often ludicrous attempts to fit the story around supposed Jewish prophecies and dwelling on the stuff about the Kingdom of God being imminent and Jesus coming back any day now.
Once the Roman authorities realised the game was more or less up for the old Empire, especially the western half which included Rome and was soon to collapse altogether, they adopted the highly autocratic, authoritarian cult of Christianity in an attempt to control the people through their religion. They had the Pontifex Maximus already in place and no doubt eager to take up the mantle of leader of the new state religion.
They could have chosen one of the others like the equally popular Mithraism, in which case we would know as little about Jesus as we do now about Mithra, but they chose Christianity. As it was, a great deal of Mithraism had already been incorporated into the Jesus myth so there was little to chose anyway and the Christians probably had a better organised power structure.
There then followed a furious campaign of seizing and burning any documents which didn't support the orthodox religion and persecution of any sects which didn't subscribe to official dogma, such as the Gnostics and possibly one based around Judas (hence him being cast in the role of a traitor by the winning sect), so the only surviving documents were those which told the required tales and those were later suitably re-edited and bound up into a single book from which we derive our entire 'knowledge' of Jesus, there being not a single shred of extra-biblical evidence that anything described in it in respect of Jesus ever happened.
It's difficult to see how Joseph Atwill's discovery of parallelism between the tales of Jesus and the campaigns of Titus Flavius can overthrow this but I remain open to persuasion.