Dear Jharkhandis, I have been reading the views/suggesions by various authours on this topic. So far what I have learned through some HINDU DHARMAGURUS (I met some ) and common hindu persons, is that the many hindus have MISCONCEPTION that "THE ADIVASIs ARE HINDUS". It should be made clear to them and others that the major Adivasis of Jharkhand i.e. Santhals, Oraons, Mundas, HOs, Kharia etc are NOT the HINDUs. They have their own religion and that is nothing but SARNA. And the SARNA religion is different from HINDU / ISLAM / SIKHISM / CHRISTIANITY. Their methodology and way of faith / worship is different from the main Indian religions. I think the remaining 24 different Adivasis (29 Nos.) of Jharkhand also follow their own FAITH. I have read many wrong articles on adivasi religion SARNA in the internet by some non-adivasi writers. All those articles are based on incompleted studies. This SARNA is entirely different from Hinduism or any other major religion. Sarna has different way of spirituality, just don't take it as PED-PATHAR (tree-stone) POOJA. But yes, with the increased intercommunity mixing (ineraction) up of these adivasis with the main stream community, many adivasis (particularly the educated and who live in towns) ha started going to temples etc. Adivasis had also started participating in hindu festivals like Dussera, Rathyatra etc. Here I am not talking about converted Adivasis. Few years back I met an "BAPU" of a SEVASHRAM SANGHA. While interacting with him, I understood that he has also the wrong perception about SARNA. He was doing some "upliftment" work, but somewhere his intensions were anti-christianity only. Then I brainwashed him about SARNA for at least 1 hour, and tried to remove his mis-conception; which was unexpected by him from a Jungle boy. Some body has told in this group that Adivasis are not hindus as these adivasis eat beef, fork etc. This is also a mis-conception. As far eating of beef, fork etc is concerned, SARNA ADIVASIs are not supposed to eat beef, fork etc. I also belong to Munda Sarna community. We have some strict system prevalent in the village/community. We don't eat beef (cow meat), fork (pig meat). We eat only the MURGA (Not the MURGI), KHASSI / BAKRA / Mutton (not the BAKRI), BHED / Sheep (Not the BHEDI). We also don't eat the animal which dies itself (due to accident / desease). We only eat only the "KILLED" animal. Eating Beef is considered as a punishable social crime. Even, killing a cow / ox is bigger crime in our community. Even if somebody kills a cow/ox by mistake, he has to do some SHUDDHIKARAN process otherwise he will be LEFT by the community. Adivasis are dependent on Agriculture and OX is the only source of power for ploughing. COW/OX is considered as most valuable commodity (AMANAT) of the adivasi family. In the occassion of DEEWALI (SOHRAI), Hindus worship LAKSHMI, but adivasis worship COWS, OXES and Buffalows, instead of LAKSHMIji. In some area, the adivasis don't work with cows/oxes (plough) on thursday. Thursday is the rest day for oxes/cows. But yes, some converted tribes are also there in the area, who eat beef / fork. I have seen several incidents of suggesting an Sarna adivasi to eat beef by converted adivasis, telling "BADKA (beef) KHANA ACHCHHA HOTA, HEALTH BANTA HAI, DAWA KA KAAM KARTA HAI." And now the situation is that some SARNA adivasis also started eating BEEF.
Even if you consider the Indian Constitution, definition of Tribe is not based on any religion. Tribes are "adivasis / aboriginals" and may be following either their ORIGINAL RELIGION (as most of the tribes have their own unique religion / faith / belief) or the CONVERTED RELIGION (may be christian / hinduism / islam etc). | 
Sincerely Yours
MMunda