Atheism [Greek (atheos) = without god or gods] is the belief that God does not exist.
If one were to say that they are “of the opinion” that God does not exist, then they more correctly would be called an agnostic.
Famous atheists include the poet Percy Bysshe Shelley and, some say, the philosopher David Hume. Hume was denied a professorship for his beliefs. However, some maintain he was not an atheist but only misinterpreted as one.
Several religions are listed as atheistic at Wikipedia, to include Buddhism, Jainism, and even Hinduism. But one has to be careful here because the concept of atheism, itself, has different meanings. Moreover, each religion has a different take on the idea of “God.” So countless hours could be spent debating whether or not a given religion really rejects the idea of God or not. This is a common problem when we get into debates about religion. No matter how hard we try to define our terms, we’re still using language. And language meanings are arguably always open to interpretation.