I don't think I ever believed it. I was conditioned to repeat the mantra but it was never real to me. "God" just seemed like all the other "gods" ... an invention, stories.
What leaving and learning more about the bible did was give me a basis and confidence that religion is just a man-made invention for power and control.
But I recognize that religion can be beneficial to some. For whatever reason, some people need some framework to operate in and some authority telling them what is right and wrong. Sometimes it's dangerous and open to abuse, sometimes it works.
I think militant atheism is a mistake - not only can it be damaging, but it's also not often convincing if the aim really is to "help" people trapped in religion. I think the point is often an excuse for someone to be a jerk to people they don't like and pretend they have some moral high-ground.