I am a strict atheist, but we cannot know. I faithfully believe that, finding no proof whatsoever, and the existence of a metaphysical/spiritual world coexisting with ours incredibly unlikely without some evidence, there is no god.

But, just like my faith in the supremacy of the speed of light, that's just my opinion, man... based on the combined knowledge of the human race and thousands or millions of experiments.

I believe in the nonexistence of God, which in my understanding makes me an atheist, and at the same time I do not claim to know about the existence or nonexistence of an afterlife. There may well be one without necessarily including a God.