...(sorry I go on about Sikhs so much, I got kinda hooked on their religion and culture, it's all just so interesting) ...
Even though Atheists and Agnostics tend to have knowledge of many religions and often more knowledgeable than Most Christians, their knowledge tends to be a mile wide and two feet deep about those religions as well. (I use the We in a broad sense for the moment knowing I might not truly be in the agnostic/atheist group) We generally have a starting point for religion that we reject and will have the most in depth knowledge of that particular religion. If we know about a second religion it will probably be one that fascinates us in some way.
For me, I know Catholicism best as my wife is Catholic so it's very recent/current. I know Mormonism fairly well since my dad is a practicing Mormon and I spent a lot of time in LDS sacrament services.
Using the things you know best as a point of reference, even as an atheist/agnostic is inevitable since our brains are wired to use something as a base point.