go well beyond simply not having a belief in God - they actively deny that God exists.

Well, eh, yeah. That's the nature Atheism. It goes even further than that - they'll also tell you why God doesn't exist by pointing out the evolution of such thinking from basic mythological animistic impulses (in which God emerged as a metaphor of Nature) to hardline religious orthodoxy (in which nature was belittled as a metaphor of God). They'll also point out that Religion was killed off by Philosophy, and Philosophy by Science; that Human Spirituality exists in our relationship with eternal infinity of the quantum cosmos which is far richer than anything we've dreamt of hitherto and all the more so for being utterly Godless.

On the latest Sky at Night (BBC's monthly half-hour astronomy slot) it was suggested that humanity has only been truly intelligent for the last 100 years or so. A sobering thought!