What is meant by "atheism is a religion"? I have heard this statement used by Christians. Atheists counter that the lack of belief isn't a religion or belief system. Is there a logical justification for the charge that atheism is a religion?

I'm looking for more than just appropriately defining the terms to make the sentence technically correct.
Atheism is a belief system which makes claims about spiritual things. Atheism says there are no gods, but so do other belief systems which no one denies are religions, such as some forms of Buddhism. Atheism contrasts with religions, rather than social or political movements like egalitarianism, anarchism, communism, so it makes sense to classify it as a religion. In a way, atheism is the religious parallel of anarchism - just as anarchists deny political structures as being valid, atheists deny spiritual beings as valid. But that doesn't mean they shouldn't be classified as political and religious systems respectively.

There are some belief systems which are harder to classify as a religion or a philosophy, such as Confucianism. Compared to that there is no doubt which box atheism belongs in: the truths it claims are solely about spiritual things, and there is no shared philosophical or ethical framework among atheists.