Definition of the word belief:

1. something believed; an opinion or conviction:

Such as a belief that the earth is flat or that the moon is made of cheese, or that god does not exisit.

or

2. confidence in the truth or existence of something not immediately susceptible to rigorous proof.

or

3. confidence; faith; trust: such as a child's belief in his parents.

or

4. a religious tenet or tenets; religious creed or faith.

So this is why I say that when one says they are an athiest they are saying that they believe that god/ or gods etc do not exist.

Believing in a lack of something is still a belief all the same.

The Atheist imho is no different than any child clinging to santa claus as real, or any other holder of a belief.

Science, philosophy, religion and personal opinion all have in common that people believe or do not believe (ie have faith in or not) the source of the statements of the individuals involved in the practice of the respected activities etc.

Historically one grows from the other, in that they are all fruit of the same tree.