Final answer:
While lacking belief in God due to insufficient evidence can be rational, claiming certainty in God's non-existence may not be, as such certainty also requires evidence and compelling argument.
Step-by-step explanation:
To address whether it is rational for an atheist to be certain that God does not exist, it is first important to consider the burden of proof. The burden of proof is traditionally on those making a positive claim to provide evidence and reasoning to substantiate their assertion. As such, in the absence of conclusive evidence to prove the existence of a supernatural deity, it is not irrational for someone to lack belief in such a deity.
However, certainty in the non-existence of God similarly requires evidence or a compelling philosophical argument, and many might contend that this certainty can be as unsubstantiated as certainty in the existence of God, especially when considering alternative explanations and the limitations of human knowledge. Therefore, atheism that is open to evidence and reasoning, while maintaining a lack of belief in God, may be considered rational, but claiming certainty in the non-existence of God could be seen as taking a position that goes beyond what is currently provable or disprovable.
Many atheists may thus identify more with agnostic atheism, which acknowledges the inability to be certain about God's existence or non-existence while maintaining a lack of belief due to lack of evidence. Therefore, a rational atheist may lean towards lack of belief rather than certainty in non-existence based on current evidence and philosophical discourse.