A commited atheist would be one who is commited to the ideals of atheism (which is both a religion and requires faith despite their argument to the contrary)..
It would entail things like:
* Not describing things using superstition but rather logic
* Not believing in god or attributing events to a god and having faith there is no god
it means committing to the beliefs of atheism, which is largely a disbelief in gods and the supernatural.