Agnosticm is a lack of belief. Atheism is a belief.
Neither of them are lack of belief. Agnosticism entails a claim that one cannot know for certain whether or not there is a God, which itself constitutes a belief about knowledge. Atheism (at least the strong form) entails the claim that there is no God, which is also a belief.
Cordially,
“Atheism is a belief.”
The burden of proof is on someone proposing a positive preposition. ie belief in a deity. Saying that atheism is a belief incorrectly attempts to reverse that burden.