Back before the days of Christ (and some days after), the world was majorly polytheistic. However, in nary a millennium, Christianity and Islam had become the dominant religions of all of Europe and West/Central Asia. I have a few questions regarding this:
Hey! You might want to cross-post this to our sister subs /r/AskSocialScience and /r/AskAnthropology. Some of the scholarship on the question of large-scale religious change is indeed historical, but there's a lot that is anthropological, or coming out of religious studies work.
The OP may have been interested in a recent published work by Bart Ehrman. Namely, The Triumph of Christianity: How a Forbidden Religion Swept the World. I cross-posted this a couple of days ago to a newly-formed sub /r/AskBibleScholars.
I, also, sent a PM to the OP concerning this inquiry.
"Evidence for the religious beliefs and practices of these people is slight, since the Indus script cannot be read and apart from the bath, there appears to be no religious structure. A certain amount of information has been derived from scenes depicted on seal-amulets and from the terra-cotta figurines found at different sites throughout the area. However such evidence is open to wide interpretation. The predominance of female figurines and seals depicting a horned goddess in association with the sacred pipal tree are generally regarded as evidence of the worship of a mother goddess who presided over fertility and birth and who may have acted as guardian and protector of the dead.
In some of the mother goddess cults of the Ancient Near East, the Great Mother who symbolized the power of fertility came also to be associated with the renewal of human life after death. She protected and revived those committed to the earth from whence this new life sprang. Inhumation was the most common method of disposing a corpse and they were buried with an assortment of grave goods, including pottery vessels which may have contained food and drink offering"
http://www.academia.edu/9961635/the_ancient_dravidian_civilization
A quick Google search is enough to back up the claims I made. If you are interested in discussion on Indian religion, one of my anthropological specializations, I'm all ears, but if you are going to shut down debate, that's not productive and really an attempt to make yourself look smarter than you are, and evidence of a bias against cultural anthropology... As I said in my original comment, I'm not basing my ideas only off of contemporary ethnography, but off the archaeological record