First of all, to most people's surprise demons are present just as they were in Jesus's day. In fact, I believe it would be hard to find someone that doesn't have a demon in them. Many people are unaware of the spiritual world, since people tend to ignore what doesn't bother them. While, I personally would go around sharing your revelation, it might be more true than you think. I would say that Atheism is of Satan, since anything that isn't of God is of Satan. However, that would not be the best way to confront an atheist about their beliefs. But any atheist that happens to read this call upon the Lord and ask him to show himself to you and he'll work in a mightily way in your life.