Why would going to church be evidence that they are good people?
Look at how people act, not what they say. When you look at crime rates, divorce statistics, and so on, atheists seem to behave more morally than evangelical Christians.
I guess my point is never underestimate "incentive-caused bias"
Take that a step farther. When you already believe yourself to be a worthless sinner who has a blank check of forgiveness from Jesus, what incentive do you have to not act like a worthless sinner?