User blog comment:LurkerLordB/Religion in a world without Christianity/@comment-1593341-20120424094112

My take, Celtic religion would dominate most of western Europe as Polytheistic religions didn't feel the need to wipe out contesting religions so the existing celtic religions would remain. The Frankish migration likely would have merged Germanic and Celtic in many areas but celtic would dominate the western coast.

Germanic religion would dominate central and northern Europe, and would influence both France and Britain, but again, France and Britain would also be heavily Celtic.

Islam came about amongst pagan religions not Christianity, but christians were the largest group of converts, so Islam adopted a lot of Christian traditions. Islam would come about, but would be closer to its original form. Without Christianity as an opposing force, it very well may become the dominant religion on earth. The idea of Salvation was the major force behind both Christianity and Islam, if the Greco-Romans reinforced the judgement of Hades/Pluto as a major part of the religion it might be able to stand up to Islam, or the warrior culture of the Germanics may be able to stand up to it as well.