Problem is that it's very difficult for Indian slum dwellers (whether they are Muslims, Hindus or Christians) to keep documents originating from 1947 (specially those who live in slums, and we know in India, many live there).
Ask slum people to show their marriage certificates (which are recent), they won't be able to show them.. People who live in one room, with several children.. without privacy.. do you think they care about docs?
India being christian majority?? The moment Christians will cross Muslim population percentage, their churches will start throwing up Ram and Sita statues from their basements.. Christians can't become a majority in Indian even in the next 1000 years.. For that, Pakistan will have to nuke India and then Europe will have to send missionaries on the wasteland once nuclear winter is over... then these missionaries should carry potential of a south asian to multiply... I believe 200 years are enough then..