The most important part of a roleplaying game is to tell apart the "real life" world of you, the player, in front of the PC, from the game world of your played character inside the PC, amphitheres.
In the medieval-oriented but fictional fantasy game world of PlaneShift, gods are a fact and do exist. Not believing in their existence would be considered a stupid opinion in Yliakum. For your character. Not for you as player.
Not believing in inevitable fate is something different. That your character may do, even though he should know that gods like Talad, Laanx, Xiosia and Dakkru do exist in Yliakum, and do have might and powers to change this world. But there are opinions that this belief is not meant by the term "atheist", but deserves an own term.