The question in the title of the thread begs another question, are 'belief system' and 'faith' equivalent terms? Atheists have to make assumptions about unanswerable questions, so in my book they certainly qualify as a belief system. But my defintion of faith (which is different than religion, also) is a communion with that greater ideal. People that believe in 'humankind' and 'the greater good' are engaging in faith. There is no provable thesis to support these concepts (belief system) and they are in communion with the idea expressed (they act/think) in such a way that supports the object of their belief... they believe they are benefactor and beneficient of this 'greater good' or 'humankind'. I do not see that phenomenon that I'm calling 'communion with the object of the belief system' within athesists: atheists do not believe they benefit from the lack of a God, and they do not seek to benefit this lack of an entity in their actions. That's not the same thing as saying they don't seek to spread a creed, which has more religious ovetones, in my book. Clear as mud, right? Let me try more concisely...

At the end of the day, atheism is not a faith as there is no interpersonal relationship (real or imagined) with demands on both parties in the relationship between atheists and their notion of non-existence of the supernatural.