@silver: No, no plans at the current time.
I don't think there were all that many defections, after all Christianity was completely marginal back then.
If you consider the New Testament to be at least somewhat historically reliable (and for the sake of argument lets suppose for now it is) most of the early Jewish converts were from the Pharisaic party. Which only makes sense, of writing about the similarities between early Jewish Christianity and Pharisees there has been no end. An argument could easily be made that the reason early Christianity was persecuted by Judaism was because it was taking so many followers from the traditional powers. And it was evidently strong enough among the Jews in Rome that it caused a riot in 49 CE, leading Emperor Claudius to issue a dispel all Jews from Rome (Suetonius,
Claudius 25.4 and yes, I do realize this is a debated passage, but I am reasonably confident if you tried to call me out on it, I could successfully defend the fact that it refers to Christ[ians]).
However, for how long can that last? I suspect that more populist religions would still eventually rise to predominate. An evolved version of some mystery religion, or perhaps Judaism or Zoroastrianism.
Excepting Judaism, without Christianity all religions, bar none, would be syncretistic, not demanding exclusive loyalty (with possible exceptions being the higher ranks of some mystery religions, because they were, well mysteries, we cant say for sure). There is no reason for one to rise to predominate because there is no reason for people to choose only one. And thats only in the cities. The countryside would hold fast to their traditions just as fiercely, if not more fiercely as they did in OTL. Excepting the golden words of poets, Zeus and other gods were regional deities associated with rocks, rivers, and local shrines, not a universal concept.
In the west, yes, but in the east I'd imagine that the knowledge would still be preserved to some degree (considering that the east was already good at preserving bits and pieces of past civilisations).
I suppose it would depend on invasion patterns and how long an eastern empire could last. I think that the easts success in preservation was do as much to the fact of a relatively stable and continuous empire than any differences in the behavior of the west vs. east.
I wouldn't say it was held together by religion (after all, it was if anything DESTROYED by religion - well, that and warlords, and eventually barbarians - and instead was held together by the civil service system adapted from the Persians).
Religion provided a thread of cultural continuity in all new conquests and motivated the learning of Arabic so that one could read the holy text, which provided linguistic unity. Leaders were endowed with religious significance, defusing the possibilities of rebellions. Religion provided motivation not only to conquer, but to incorporate, they were after lasting conversions, which would naturally lead them to undertake projects to ensure their religions lasting hold on the area, projects of both religious and civil nature. In addition, their leaders were elected, not succession by blood. Taken all together, without Islam, I would argue, there would be more rebellions, earlier, combined with succession wars. Thus, the Arab Empire would be just like other barbarian empires, fast growing, initially successful, that were victims of their own success and quickly imploded within a generation or two.