Atheism is not a religion, but it is a world-view, and as a world-view it takes on many of the attributes of religion.
1) Seeking Converts
Atheists want everyone to believe as they do and are continuously trying to convert people to their world-view. If you don’t agree, think of Bill Maher, the late Christopher Hitchens, Richard Dawkins, that guy with the bow-tie who does kid’s shows, and all the guys on Youtube.
2) Controlling Politics
Atheists want their people in office to create policies which favour the atheistic world-view.
3) Punishing the Heretics
If you are not an atheist, you are not welcome in their society.
4) Preaching Salvation
Salvation comes through atheistic/humanistic moralism and science.
Religious people and atheists do all these things alike. This is why people will say that atheism is a religion. Sure, atheists don’t worship a super-natural being, but they do worship something. We all worship something.