An agnostic doesn't "doubt God...he just doesn't know.
I guess it depends on which agnostic you speak to. A woman from work is one and she says "she's believes in something bigger out there, just not God". Now she is a "Jew", who celebrates ALL the Jewish holidays. I asked her if she didn't believe in God, why does she do it. She said "it's tradition".
I told her, in a nice way, that she was just an atheist without b@!!s.