Some religions are secular- Atheists should oppose these too

[Darkstar]

Even that isn't entirely accurate; people can blindly believe there is no God simply because that's how they were raised. Skepticism is about questioning everything and if you just blindly accept that God is bullshit without questioning it, you aren't really skeptic. I've always said, I'd rather have a discussion with an intelligent theist who has put intelligent thought into their beliefs than an atheist who is just blindly following what his parents told him to believe and doesn't know why he's an atheist.

Technically, you would not necessarily need to be a skeptic about the existence of god if you were raised without religion in the first place. However, as theism is making the claim that god exists, atheism is the logical default position. So, while skepticism is technically not required for atheism, the example you used would be more accurate (and certainly more common) if the theist and atheist role were reversed. Not to question the assumption that an omnipotent and undetectable being exists is a lot harder to understand than not questioning the assumption that he doesn't.

Regardless, I take it that by skepticism you imply critical thinking skills as their foundation. If that is the case, then a lack of skepticism in an atheist would indeed make for a poor discussion. As for a theist putting intelligent thoughts into their beliefs...it can be done, but only very carefully...