The Lebanon thread had me thinking: Over the last 150 years, the trend throughout the Middle East has been for the area to become more and more Muslim. Christians began emigrating from Lebanon and Syria in large numbers around the turn of the 20th century, and now large numbers of Copts and Chaldeans are moving to the U.S. At one point, IIRC, Palestine actually ranged up to 30% Arab Christian - almost all of whom have become part of the Palestinian diaspora now. And of course, the long-standing Jewish communities outside of Israel are now largely gone.
Could we see, with a POD later than 1500, the reverse? Some sort of mass migration of Muslims out of the Middle East to somewhere else? Please note, I'm not asking for neo-crusaders to ethnically cleanse the area. I'm also not asking for some sort of mass conversion.
But would it be possible to have a substantial enough migration to have roughly the following percentages Christian?
Lebanon (in the modern sense) - 60%
Palestine - 50%
Syria - 30%
Egypt - 20%
Jordan - 12%
Iraq - 7%
Plus, of course the Jewish communities in the Middle East pretty much remaining as they were pre-Israel
eschaton
I think to get widespread Muslim emigration with the result that the Christian population is proportionally dramatically increased would be earlier and more doctrinaire foreign control of the region. By more doctrinaire I mean that new non-Muslim rulers, would have to be Christians really as can't see anyone else having the power, seek to encourage their own faiths far more than the European colonisers did in OTL. Not necessary forced conversion or anything like that. You might have:
a) An application of the sort of rules early Islam applied to their conquests with extra taxes and relatively minor discrimination against the Muslim minority. [Since this also means giving the Christians a majority of military power but imposes military responsibilities for them this would also have impacts].
b) There is a view amongst some elements of the Muslim community that Muslims must live under Islamic rule. This was not important during the early centuries when Islam was expanding but did cause some debate during early set-backs, such as the Byzantium re-occupation of much of Syria in the 10-11th century and the Spanish reconquest. [In the latter this was irrelevant eventually because the Muslims were forcible expelled anyway but not thinking of this here].
Factor b) is less likely to have a significant impact unless there is a clear discrimination against Muslims or as Pasha said better opportunities elsewhere. I doubt that many people will give up their homes and lands just to be under Muslim rule unless markedly better opportunities elsewhere and/or conditions are getting very bad.
You might get the doubling of the Christian proportion of the population in Egypt and possibly also the figures suggested in Iraq. [Since there were ancient Christian communities in that region]. Also the Lebanese target. Not sure about the others.
However it would be difficult, especially since given that the Muslim community has a long history of power and dominance there would probably be continued unrest, even with pretty moderate treatment. [That gives opportunities for population movements but not really the voluntary ones your thinking of].
Also, maintaining the Jewish population would be a problem. Until the modern period which discriminated against the Muslims tended to treat Jews better than the Christian parts of the world. Therefore it would need some fairly significant changes in the new ruling group to make a more moderate view to them I fear.
Steve