They convert to Christianity primarily because of the material benefits ( i.e ricebags for civilians ) and these Missionary groups tend to always be fronts for CIA/NSA/Glowniggers, so for your average naxal/maovadi bandit it also means a source of funding and equipment.
The tribal converts also aren't true believers, probably get baptized for those gibsmedats and some days later worship some magic rock in the jungle and then offer a plump goat as a sacrifice to the river-spirit the day after, just as their forefathers have been doing for thousands of years.
ofc the "Missionaries" purpose is to weaponize these tribals as a thorn in the side of the indian govt in naxal/maovadi forms, not actual religious belief
Aside from the Protestant ones, in the Catholic church there are adherents of "Liberation Theology" or crypto-Commies like i call them, the most (in)famous among them is (((Stan Swamy))).
"Liberation Theology" is a product of mixing Christian precepts with Communism, it was first created in Latin America during the Cold War period( since directly opposing Catholicism wouldn't endear the natives to the cause of the "Revolution" like how the orthodox atheist commies would ) and focuses on muh virtous poors, muh evil rich, and other such commie trappings.
TLDR a Commie in a Cassock.