in any capacity that they'd like, seeing that there are no real rules on how to not believe in a god.
One of the great things about being a non believer is that we never have to compromise our feelings and ideas to fit any sort of preconceived code of rules for behavior and thought if we don't want to. If an Atheist feels a kinship with those who worship nature, then by all means they should feel and express that if they'd like. Being an Atheist means being free in thought.
I know that we're mostly talking about Paganism before Christianity, but for the record I am much more accepting of modern Paganism than Christianity for the following reasons:
I think the reason some Atheists feel a kinship of sorts with modern Pagans/Wiccans because we share some similar issues and concerns that come with not worshiping a patriarch god. We have in common that we place our focus on the natural world, and we just want to be left in peace.
In the vast majority of cases, Pagans don't persecute anyone. They don't want to convert anyone. They don't tell you that you are going to hell. They don't care that you don't believe in a god.
Pagans usually don't worship actual "gods", and are closer to our thinking than other religions.
Another reason I am much more friendly towards Pagans than Christians is because they may be in a later portion of a journey towards free thought. Today's Pagan may be tomorrow's Atheist, and I don't want them to think that Atheists are cold, uncaring and not friendly. Actually, I used to be a "New Ager" before I became an Atheist. That's how I know this.
