I'll be repeating myself in any response.Here's the list you provided:
In Christianity alone there is not a whole lot of commonality in the above, outside perhaps the social aspects of gathering together. Excluding "the supernatural" for the moment, all of the above apply to things atheists share in common as well, as they are all social and cultural values which anyone, believer or non-believer participate in the culture at large. Atheism in the West, in all honesty, based upon the fact they are part of Western culture can really, actually be called "Christianity without God". Chew on that one awhile. I look forward seeing the flack from that one!
- many regularly meet at their church or equivalent
- morality
- the supernatural
- purpose
- prohibitions, etc.
Though it's obvious that term will not be received well by most, it is a valid statement if you honestly think about it in the light that we all are part of the general Western ethos which has been informed through Christianity in the West. No one escapes it. Even atheists, in their rejection of God, still have the same basic moralities within the common ranges of conservative to liberal. They still see the world through the same sorts of general frameworks, not launching off into some truly foreign modes of thoughts such as you would find in cultures we did not grow up in, such as Eastern cultures. These are not merely ideas and beliefs, but entire ways of thinking themselves that mark cultures from one another. Therefore, Western atheists are in fact in general part of the Christian heritage of values and modes of thinking, irrespective of specific beliefs about gods and science and whatnot.
Anytime you have people who have relationships with one another, cultures and values become conferred upon those who are part of them. Don't believe for one second this has anything to do with beliefs in the supernatural. It has to do with being humans and social animals. Atheists are that just as much as Christians are. Group dynamics apply to them equally, and you'll see the same things going on, even it it is operating at different levels. It happens in children, it happens with adults, it happens with men, it happens with women, it happens with believers, it happens with atheists, and so on. No escaping it.
You are mistaking Christianity as being only about teachings of the Bible. It most certainly is a whole lot more than that, and in fact that's really incidental to what makes it what it is. What makes it what it is is people in relationships with one another, bringing their values to bear with others and co-creating the religion. The "Bible" then simply becomes reflective of their shared beliefs and values in how they apply their interpretations to it. The exact same thing happens with groups of atheists, and humanists, or Masons, or Lion's club, or Rotary clubs, and so forth.
You seem to assume Christian actually has God telling them what to do and they aren't part of creating what it is for themselves. It is nothing but what they create for themselves, and the exact same forces are going to be at work with atheists who have any sense of community with other atheists, be that in local gatherings, or here in the Virtual World. Forums are in fact places of social gathering and interactions and cultures. Atheists do have that. You are participating in it right here, right now. It's right in front of you, and all the forces are in play creating shared beliefs and values. There's no escape.
We've beaten this poor dead horse too much already.