To clarify, 1inC, you have put yourself into an untenable position. Either you must admit that it's possible to be good without following god, or you must rule that a moral action is immoral depending on who does it.
First case: If telling the truth is moral, and an atheist tells the truth, he is being moral. Therefore, belief god is irrelevant to morality and your hypothesis is disproven.
Second case: If a Christian tells the truth it is a moral act, but while an atheist tells the truth it is an immoral act. Morality, therefore, rests not in the action but in the religious beliefs of the person performing the action.
I know Aquinas could defend either position, but seriously doubt you're up to the task.
(edited for redundancy)