I was born an atheist. Fortunately my parents, and their parents let me decide what I thought of religion, while always explaining that they thought it was complete nonsense.
As a result I never took faith seriously despite attending Church of England schools, as there was never any evidence offered for the claims of the bible. I became less passive during adolescence when I reflected on the damage religion does to civilisation, and after September 11, my father and I became avid followers of the various luminaries of the Atheist movement.
There is no need to explain why I am an Atheist – it is my natural state and it falls on the religious to convince me why I should be otherwise.