Pope Francis on Paris Attack - "one who throws insults can expect a 'punch'"

[Slander and/or Libel]

There is nothing wrong with insulting religions either. Insulting a religion is not the same as insulting believers.

You couldn't be more wrong about that, and I feel the same way in regards to non-thiestic religions such as atheism and humanism.  That said, insulting religious leaders is in general not the same as insulting their religion or their followers.  I say in general because in a very real sense, Muhammad, Jesus, Gautama , etc. have transcended being people who are leaders of a religion to becoming symbols of that religion.  That's a distinction that many secularists have difficulty grasping the concept of, let alone acknowledging the validity thereof.

Atheism is a religion?

Yes.  A religion is an organized collection of beliefs, cultural systems, and world views that relate humanity to an order of existence.  While most religions contain some supernatural aspect to them, it isn't a requirement.

Given the heterogeneous nature of the beliefs of atheism (apart from generally all agreeing on the God question), it's difficult to say that "atheism" as a broad category fits the definition you give.