What if, part 2
(Sidenote to Knarf: the Romans did all that fantastic engineering using only whole numbers?! Wow!)
1st Nudge. Following the execution of yet another Judean revolutionary, a heated debate broke out among his followers. Paul and his faction argued that anyone was allowed to join the movement, while James held that only observant religious Jews could receive the teachings of the Nazarene. In our timeline Paul won, but in this alternate timeline James won, with the result being that "Christianity," with its emphasis on strict dietary laws and adult male circumcision (ouch!) remained an obscure pacifistic strain of Judaism, not much different from any of the hundreds of other tiny cults that fluorished in the backwaters of the Roman Empire. The Romans still found Judea a troublesome and revolt-plagued corner of the Empire, which didn't settle down until they demolished the Temple in Jerusalem — and for that matter, most of the rest of Jerusalem, too — dispersed the surviving Judeans throughout their empire, and ultimately, renamed the whole region Syria Palestina, in hopes of erasing even the memory of the old Hebrew Kingdom.
2nd Nudge. Maybe Diocletian's political reforms took hold, so that Constantine never had the chance to rise to power? Or maybe it's a simple as, absent Christianity, Constantine never had the religious vision which inspired him to overthrow his co-emperors? In any case, in the early 4th century, Rome remained a tetrarchy, which meant Constantine never got the opportunity to split the Empire and build a new capitol city in Byzantium. (Poof! There goes the entire Eastern Orthodox church, along with most of Russian history up to the Romanov dynasty!)
3rd Nudge. In any case, when the Huns appeared in the 5th century and drove the Goths, Franks, Lombards, and such south, they found Roma to be a strong, unified, and well-defended empire, not a tottering wreck, and so they entered it as grateful refugees, not as looters. Within a century the Goths et al had been thoroughly assimilated, via the traditional means of enlisting in the army and serving overseas, and so when a certain Arabian bandit chieftain started raiding caravans and gathering followers in the 6th century, Rome was able to treat it as a local law enforcement problem. With typical Roman efficiency they sent in the Legio VI Germanicus with orders to solve the problem, which the legion did with typical Germanic subtlety by killing every Arab they could catch, leveling Mecca and Medina, and poisoning every well and oasis they could find before withdrawing to Alexandria. As a result, the Red Sea remained a Roman lake, and by the late 7th century Roman sailors had made it as far as the Bay of Bengal to the east and Lemuria to the south.
How are we doing so far?