What was the first religion?

Answer 1

The "oldest" existing religion is Judaism. Not Hinduism.

Judaism has its roots to the Books of Moses, to the book of Enoch and the Book of Adam and Eve. These were the first written accounts of the creations of God.

Answer 2

Zoroastrianism was probably the first of the monotheistic faiths closely followed by vedic hinduism with which it shares numerous details. Although the very first faiths were almost certainly animistic, that is, revolving around the worship of places and ancestors.