Some census information

Since this is important for the joint electorate + reservations issue:

Undivided India
Percent of Total Population in Each Religion 1881-1941

                  Hindu    Muslim

1881              78.09    19.97

1891              74.24    20.41

1901              72.87    21.88

1911              71.68    22.39

1921              70.73    23.23

1931              70.67    23.49

1941              69.46    24.28

From Davis Kingsley, The Population of India and Pakistan, ~1951.