This very interesting article in the Boston Globe addresses a very interesting question to which I had not previously given much thought. How is it that a religion that appears to be in no way evangelical (Hinduism) managed to become a religion of 900 million people. The gist of the article appears to be this. Basically, a single, the idea of a single homogeneous Hinduism is a relatively new: it is to some extent a reaction against Islam and Christianity, both of which have come to India and both of which are evangelical, and also a product of the institutions of the British empire, which liked and was more comfortable with the idea of a "single religion", than what actually existed in India at that time, was a very complex and varied set of mutually tolerant faiths. The empire recruited Indians from a certain elite (partly for administrative purposes, partly because it found their culture interesting) and spread a Brahmin culture that much of India was not previously familiar with. The result was also to spread a sense of India as a single nation with it. Thus, a unitary Hinduism is a recent development, and may be a force (and tool) for nationalism, the way Christianity was for the Roman empire.
Update This somehow got garbled when I first posted it. It has now been fixed.