Atheistic is the adjective for things that belong to, or sound like, the belief that God does not exist: an atheistic argument, atheistic views, an atheistic book. It is more essay-like than atheist. You are more likely to meet it in writing than in chat. Though actually, English speakers often just say atheist even before a noun (“an atheist philosopher”), so atheistic can feel a little stiffer, and a little more about the idea than about the person. It still points to that same belief; it does not mean “against churches” or “rude about religion” unless the sentence adds that.