If Adam and Eve were the first humans on earth and taught their children Christ’s gospel, why is Hinduism historically the first religion?
It is interesting that you use the term “historically”. Any historian will tell you that we don’t have all the answers. We have the best guesses. Our guesses are only as accurate as the evidence we currently possess. Even then the evidence is subject to interpretation and subjective characterization.
HINDUISM CLAIM AS OLDEST RELIGION
Hinduism as a formal set of beliefs and practices didn’t come into existence until around 500 BC. Precursors originated earlier than that stretching out a few millennia (as you say around 3000 BC). But that is only given that recognition to allow Hindus to be able to stake their claim as the oldest religion. Many outside the religion would not consider the religion to be the same.
LDS RELIGIOUS GENEOLOGY
The Church of Jesus Christ of Latter-Day Saints is one of the newest organized religions on the face of the earth. Yet we believe that we only restored the earlier faith of Christ’s time that would make us pre-date Catholicism. Prior to that we must also acknowledge that Early Christianity sprung from Judaism as a fulfillment of the Law of Moses. Yet Christianity itself is considered to have started only with Christ’s mortal ministry.
Judaism was considered to have started with Moses (circa 1350 BC) because that was when The Law (Torah) was given. And The Torah is the definition of that religion. Yet the children of Israel of Moses’ time claimed ordinances and observances from the Abrahamic Covenant which would also be considered a different set of beliefs. The Abrahamic Covenant began with, well, Abraham. He is thought to have lived around 2000 BC. Yet he paid tithes to Melchizedek (King of Salem) who was the last of the Patriarchs per the Patriarchal Order from Adam on down.
Each dispensation had different covenants (or set of observances). Thus outsiders would easily consider these to be different religions. But our system of beliefs says that it was the same God whom we worshipped throughout all these dispensations that links us all together as the oldest religion.
TRULY OLDEST RELIGION
To be fair Zoroastrianism is considered the oldest continuing religion which originated around 1500 BC. This one doesn’t really have any ifs, ands, or buts. They really have been around for 3500 years with pretty much the same system of beliefs. But it didn’t become a “big” religion until much later.