No one was converted by coercion ... no one put a gun to someone's head to convert. People convert when either they believe in the new order of religion or they think that the benefits of being a christian far outweigh those of being a Hindu.
We Hindus do not let shudras near the temple ... in some places they can only see the deities from a distance and that too not at sunrise or sunset ... for such Hindus, converting to christianity or any other faith is no brainer.
