Another exception to this rule is as follows:
I before E, except in God.
"What?" you ask. "'God' doesn't have an I or an E in it!" You are, of course, right. However, many words which refer to the deity, theism, or atheism do indeed have such letters. If you were paying attention, you might have seen three in the previous sentence.

In other words: It's "deity", not "diety" -- and "atheist", not "athiest".