My child is an atheist. How did this happen when I raised her as a Hindu Brahmin?

Such is life. We can only guide children and do our best that they follow whatever we do. We always want our children to follow us because we think we are right, and set ourselves as benchmark. We should introduce our children to all religions and cultures and should leave it upon them what to do. Let them be free in their disposition, only then they will discuss their deepest fears and joys with you. If you will put the pressure of expectations upon them, they will be a closed heart in front of you. I suggest, accept their outlook and choices in life, and if there's something that you really think is not fine, discuss with them. Give them logical reasoning than emotionally binding them. This way, your kids will be open to discussions and buying your point of view.