It does absolutely depend on how you define god.

If you define a god as an omnipotent, and omniscient conscious being, then one absolutely can be an atheist with regards to it with 100% certainty since this definition of god is a paradox. If god is all knowing, then he knows the future, and if he knows the future and attempts to change it, he would have been wrong about knowing the future and thus not be all knowing. If he can't change the future due to his omniscience needing to be kept in check, hes therefore not all powerful.

A god defined as such cannot therefore exist.