As requested from a certain Muslim I was debating with([link]), here is my argument:

Islam was not the first religion ever practiced by Mankind. It wasn't even the first monotheistic religion ever practiced. Judaism was the first successful monotheism, and even that was after the earlier Hebrews' henotheism.

The first religion that we can find evidence for was neolithic Animism, practiced by hunter-gatherer times predating the invention of Agriculture.

Islam is relatively new on the scale of religion.

You want proof? Visit your local Museum some time. Better yet, watch a documentary on the History channel. You'll be surprised what you find.