Everyone has their beliefs, their faiths, whether or not you believe in a god(s), a force of some mystical origins, or a natural force it doesn't matter much to me. Everyone has their "god", everyone has their "religion", whether or not it is in the contemporary, common definition of "religion" there is still faith in the mind of the atheist/agnostic.
To a Pagan, it isn't that difficult to call what atheists believe in a "faith". And yes, I know, atheism is a statement against belief in a religion, not a religion. However, atheists have minds that hold non-religious beliefs as well. Most atheists believe in SOMETHING, whether it is science (materialism), themselves, a nation (patriotism), their families, their friends, theories, laws, facts, figures, money, power, fame, television; everyone, including atheists, worship someone or something.
An idea, a person, a place, a country, a group of people, themselves (their ego). Belief doesn't stop at the temple doors.
I should say, that though many of you are likely to protest what I am saying (which has been said many times in many ways) you can't escape the truth of it.
Besides, it isn't as if all religions have gods; I'd hardly call Taoists, Confucians, Pythagoreans..etc.. atheist.
No matter who you are, there is an end to your ability to prove yourself correct. At some point you must take a leap of faith, backed by no evidence or logic. There is no way for you to "prove" anything true, after all. The best scientists in the world can only say they are sure of any fact or law "99.9%", there is only disproof. At some point you are saying to yourself "this is true because I feel it to be true, it seems correct".
You'll come across this more often than you'd like to think, especially if you truly examine your belief structures.
So let's not say what would "shake your atheist/agnostic faith"; let's say what will make you expand your current faith to encompass traditional religious belief.