It occurs to me that it doesn’t make sense for an atheist to say “sure as hell”. I mean, you wouldn’t say “I’m as sure that Mordor exists that I ain’t volunteering for this assignment” (note to self: try using the phrase “sure as Mordor” and see how it goes over).
From a purely factual standpoint, it’s much better to say “sure as shit”, since shit is known to exist. Unfortunately, the use of that phrase isn’t always appropriate. The best I’ve been able to come up with so far is “sure as Shinola“, but I’m sure you can do better. Discuss in the comments.