Atheism is a belief; it establishes a single absolute conclusion to the same question.
Agnosticism is not a belief; There is no conclusion drawn because of a lack of supporting evidence. When god can be proven or dis-proven mathematically or personally then a decision will be made.
Other religions are beliefs; drawing absolute conclusions without verifiable explanations.
The difference between atheism and agnosticism is artificial and of recent origin.
Most people who are not religious now, are not religious for the same reason: There is no proof that a god is required for the universe to exist.
Any reasoning person, when shown incontrovertible evidence of something, must come to accept that same something. It's how science works.
Show me simple, repeatable and verifiable proof that god exists, and I'll be a believer.