In the United States, most people believe that you must be religious to be moral, and further that atheists are not only amoral but also arrogant.
Religion has a set of rules to follow; without adhering to a religion, you have nothing but yourself to guide you. Often, religions teach that people are inherently sinful—inherently bad. Following your own moral inclinations would therefore create a selfish, malicious ideology. This is where the idea that religion is a prerequisite to morality comes from. When you reject the guidelines of religion, you’re either choosing to follow your own ideas or you’re outright rejecting any restrictions on your behavior. Either way, you’ve rejected any oversight. The only reason to reject guidance, the logic goes, is that you’ve decided you know best, in which case you must think yourself perfect. Believing oneself perfect is the result of an arrogant personality, which confirms the idea that nothing good results from rejecting religion.
The fault in this logic is the assumption that rejecting one type of oversight—religion—means rejecting all restrictions on your behavior.