It's because they've been brainwashed into believing that all atheists are sinners, thieves, adulterers, rapists, or worse. They are literally incapable of imagining a person being good with God. Therefore anyone who claims to be an atheist but is actually a good person must clearly be lying or deluded. No other explanation will satisfy those trapped in this way of thinking.
Religious leaders perform this brainwashing because the very idea that morality can come from somewhere other than religion is a threat to their business. People would leave religion in droves if they found out the truth.
Like when Danish priests asked the athiest movement in Denmark why there were only religous foundations that did charity.. The short answer was.. "Why do we need a new foundation when we work freely with you?"
Then it went silent and it was never heard from again.
Religous people/institutions should be sacked so they dont have political power.
Either you choose the church or the political world.
Cant mix the shit together.