Nehru signed a treaty with a Christian Missionary named Verrier Elwin. According to this treaty, Hindu Sadhus were banned from entering Nagaland. It was under Nehru's rule that Nagaland became a Christian majority. Today, Nagaland is 88% Christian as per the 2011 census.
Verrier Elwin was converted to Hindu under the influence of Gandhi in 1936. In fact, he was delicensed by the church of England when he refused 2 take gospel 2 tribes. But that was probably a facade, he kept working on segregating tribals from the mainstream Hindus.