This process was complete, even Spencer admits, within 60 years of Muhammad’s death. In this, Islam beats Christianity by a few centuries, since it took about 300 years for Christians to settle on what would and would not be Scripture, and another several centuries to form a rough consensus on whether Christ was or was not identical with God the Father. Judaism took something like a thousand years to develop its traditional form.
I just don’t see how evidence for the evolution of Islam, no matter how interesting on its own, proves the case that Muhammad and Islam are a fabrication designed to impugn Christ.
…As we have seen, the contemporary chroniclers from the lands they invaded called them “Hagarians,” “Saracens,” or “Taiyaye.” The invaders referred to themselves as Muhajirun, “emigrants”—a term that would eventually take on a particular significance within Islam but that at this time preceded any clear mention of Islam as such. Greek-speaking writers would sometimes term the invaders “Magaritai,” which appears to be derived from Muhajirun. But conspicuously absent from the stock of terms that invaded and conquered people used to name the conquering Arabians was “Muslims.”
But does this mean anything?
Take the case of English. The word “Muslim” does not enter the English language until after 1610 as a noun, and 1777 as an adjective, replacing the earlier “Mussulman,” which dates back only to 1560 at the earliest. Prior to this, and down to the 1960s, the usual term was Mahommedans or Mahometans, but even this only dates back to the 1520s. Prior to this English speakers (as with most Europeans) did not differentiate Islam (the faith) from the ethnic groups that had adopted it. The reason for this is because the West believed that Muslims were heretical Christians since Muhammad preached the veneration of Jesus as a prophet. For medieval Christians, these were Arabs with heretical views, not an organized faith with composite ethnic groups. Surely Spencer does not mean to suggest that Muslims did not exist at these late dates just because the English failed to extend the courtesy of calling them by their preferred name.
As an analogy, consider English views of Germany. Down to the present day, English-speakers call Germany after its Latin name (Germania) despite the fact that the Germans refer to themselves as Deutsche and their country as Deutschland. We call the Greeks after their country’s Latin name (Graecia) despite the Greeks referring to themselves as Hellenes and their country as Hellas. And one would be hard-pressed to find an English-speaker who refers to Japan under its native name of Nippon.
I could go on, but I would also note that Spencer himself refers to the Greek writers of the early Middle Ages as Byzantines despite that term being an Enlightenment invention. In fact, those writers did not call themselves Greek, either. That was a Western term foisted upon a people who, until the Ottoman conquest in 1453, referred to themselves as Romanoi—Romans—since their empire was the legal continuation of the eastern Roman Empire.
The point is that the fact that the Byzantines did not in the first decades of Islam record the Arabic terms used for the religion is not evidence that these terms did not exist. It would, to my mind, be rather shocking if the various nations and tribes converting to Islam suddenly abandoned their ethnic names, erased their tribal affiliation, and called themselves exclusively Muslims. After all, the Europeans didn’t call themselves Christians only—they were Romans, Franks, Visigoths, etc. The religious name existed for describing religious functions, and the national and ethnic names for political and legal functions. The Byzantines did not call the Persians “Zoroastrians,” and there was no particular need to identify Arabs as Muslims. In ancient empires, religion and ethnicity were not synonymous since one could be a subject and a believer and be of any ethnicity. It is only later, in the early modern era of nation-states and official churches, that scholars began to conflate citizenship, belief, and ethnicity as three facets of one identity. But in the imperial sense, this correlation was never complete. This is why the Ottomans were called the “Turk” down to the end of the Ottoman Empire and not “the Muslims.” Religion and ethnicity were not synonymous.
As a final point, Spencer is writing about a period only a few decades after Muhammad’s death. At a comparable point after the death of Christ, Christians still thought of themselves as a sect of Judaism and were only rarely called Christians, first at Antioch (Acts 11:26), but even this term was a Greek interpretation of a Hebrew term for the followers of the Messiah. According to the New Testament and the Church fathers, “Christian” was originally a derogatory term used for a people who called themselves Nazarenes. Outside the Christian tradition, the name is first attested in Josephus’ Antiquities of the Jews 8.3.3 (c. 95 CE), Pliny the Younger's letter to Trajan of 111 CE, and Tacitus’ Annals 15.44 (c. 116 CE)—the same 60 years or more after the death of Jesus that the first mentions of "Muslims" occur after the death of Muhammad in non-Islamic sources.
Tacitus says the Nero blamed the Christians for the Great Fire of Rome in 64 CE. Obviously, we can only conclude with Spencer that Christians did not exist until Nero invented them.