Did Constantine create Christianity? - this thesis seems to think so -
http://www.mountainman.com.au/essenes/thesis.pdf
It's about as plausible as
this.
The thesis is ingenious but it doesn't even begin to make its case. The author spends a lot of time dismissing the archaeological evidence for pre-Constantinian Christianity. I'm not competent to evaluate the success of this dismissal. But even if he's correct, he's not entitled to conclude that pre-Constantinian Christianity didn't exist. He still needs to explain why his theory is a better explanation for the literary evidence, and he doesn't do that. The overall argument is along the lines of "X is unproven, therefore Y" and "The evidence is consistent with Y, therefore Y" - and these are classic conspiracy theory ways of arguing.
The closest he comes to a positive argument for his theory, as far as I can tell, is his claim that the first references we have to Bibles (Old and New Testaments) bound in a single codex are post-Constantine. He claims that if Christianity were older than Constantine we'd have references to such Bibles from before. This is a staggeringly weak argument. First, it's not obvious why early Christians should be expected to have bound the Bible in such a way, except that we in modernity are used to thinking of the Bible as a single book; he's anachronistically projecting modern expectations onto antiquity. Second, such volumes would have been very expensive, and pre-Constantinian Christians were not known for their wealth. Third, codices were not very popular before Constantine anyway - they only became as popular as scrolls at the beginning of the fourth century.
He wants us to believe that, on Constantine's orders, Eusebius of Caesarea masterminded the forging of all "pre-Constantinian" Christian literature, from the New Testament onwards. This is obviously an intrinsically highly implausible claim which requires some pretty good evidence to support it, but as I've said, he gives barely any evidence at all. He doesn't state who he thinks actually carried out this work of forgery (surely not Eusebius single-handedly?) or how they did it. Most damningly, he does not (and cannot) explain why, if all the pre-Nicene literature was forged on Constantine's orders, it doesn't reflect Nicene Christianity. For example, why would the texts fraudulently assigned to Tatian of Syria and Tertullian teach that there was a time when the Father generated the Son, which is contrary to Nicene Christianity? Why do we not find, in Tertullian's writings, an insistence on the power and authority of the emperor as the true representative of God, as found in Constantine's own
Oration to the Saints? Why, indeed, do Tertullian's later writings attack the Catholic Church and support Montanism? Why would a Eusebius commissioned to support a cult invented to back up Constantine's military power - a feature of his rule that the author repeats constantly - have invented Tertullian's
On the Crown, which denounces the military and insists that no Christian would ever join it? Why would Eusebius and his team invent this stuff when they could have invented stuff that supported Constantine's own views much more consistently? Why would they have written the Book of Revelation with its stinging attacks on the Roman Empire when they were trying to create a mythology to support the authority of that very empire? And so on and so on for pretty much every pre-Nicene text.
Rather extraordinarily, the author is prepared to accept that Origen was a real person who really wrote commentaries on the Hebrew Bible, but thinks that all of his commentaries on the New Testament (and presumably his voluminous doctrinal works) were forged by Eusebius and co. No-one who has actually read Origen could possibly think this, because Origen's Old Testament commentaries are full of distinctively Christian material. The forgers would have had to rewrite them quite extensively. Moreover, absolutely no textual evidence is given to support the claim that (say) Origen's commentary on Genesis was written by a different person from his commentary on Matthew. It's simply asserted because the theory demands it. That again is classic conspiracy theory thinking.
Interestingly, the author is familiar with Rufinus' claim that Origen's works were interpolated by heretics, but that doesn't support his case at all. Rufinus thought this because Origen's works contained ideas that didn't match Rufinus' own notions of orthodoxy. The "interpolations" that he removed from his translations were not the references to Jesus (or whatever) but claims about the resurrection body and the salvation of demons. Rufinus certainly didn't think that most of Origen's works were outright forgeries - if he had, he wouldn't have spent so much time translating them.
I rather enjoyed the section where the author ingeniously asserts that Arius was condemned not for teaching that the Son is not consubstantial with the Father but for trying to expose all of this fraud. He makes out that the Arian catchphrase "There was a time when he was not" refers not to the divine generation of the Son but to the invention of Jesus by Constantine. Unfortunately even the most casual glance at Arius'
Thalia shows that Arius believed not that Jesus was a recent fabrication but that God the Father created the Son before the beginning of history. There is no textual evidence at all to support the author's interpretation of Arianism, and he doesn't try to give any. Moreover, he's obviously unfamiliar with recent scholarship on Arius, which has overturned the old view of him as a "logician" - which the author repeats here. He rather extraordinarily accuses Constantine of having Arius poisoned and says nothing of the fact that Constantine planned to rehabilitate Arius, something obviously inconsistent with his theory. He also has nothing to say about the continued existence of Arianism or the many, many councils that were called to deal with it between Nicaea and Constantinople. Were these later Arians also trying to expose the literary fraud? Or had they misunderstood Arius, just as everyone today supposedly has, and were genuine Christians where he wasn't? The author has nothing to say about this. He also has nothing to say about the fact that Constantius II was largely sympathetic to Arianism and that the councils he called produced more or less Arian creeds that were hostile to the creed of Nicaea. How could that be, if Constantius was simply following in his father's footsteps of building on the fraud perpetrated at Nicaea?
The author also does not explain (or even acknowledge) the existence of movements such as the Donatist church, which rejected Constantine's authority and rejected the rest of the Christian world for allying itself to him. If Constantine invented Christianity, how could there be sizeable bodies of Christians who regarded him as an opponent of true Christianity? The author thinks that all pre-Nicene church history, including the persecutions of Christians at the hands of Diocletian and other emperors, was faked. But if that is so then the pre-Nicene history of Donatism, including its origins in disputes about the authority of bishops who capitulated to the authorities during the Diocletian persecution, was also faked. Why would Constantine and his lackeys invent the history of a Christian movement hostile to themselves? At what point did Donatism actually come to exist, and how did it happen?
So in short, the whole thing's a conspiracy theory that falls apart as soon as you look at the details even of fourth-century church history.