Not only you majority of non-mulims think that islam started 1400 years ago and before that it was not existed....
According to Islam mankind was started by Adam & Eve and they were muslims. We dont beleive in darwins THEORY of evolution. Now when & how was hinduism started and how it is oldest religion is yet to be proved.
And we consider entire human belongs to Almighty Allah......
Let me clarify one thing these kind of belief also in chrisitianity also but their concept is totally different thats why they too are considered KAFIRS like hindus....
