Atheists: Why do Christians say we have to start believing in God to believe in God? Isn't that a paradox?

I don't think its a paradox more of a circularity really. The definition of a paradox is "A paradox is a true statement or group of statements that leads to a contradiction or a situation which defies logic and or intuition" . I don't think that qualifies as a true statement which leads to a contradiction.