Religion-based morality is usually grounded in the authority of a god, although there can be a more dispersed basis for that authority, as there seems to be in some Eastern religious traditions. From a Christian perspective, right and wrong conduct is fully determined by God. Without God's authority to motivate conduct, people can misbehave in any conceivable way without fear of punishment or loss of reward for good behavior. God figures heavily in their calculation of the best course of action in the future.
When confronted with an atheist, people of religious faith are understandably concerned. They now face someone whose basis for morality is largely missing. Such a person would seem to pose a worse threat to society than someone who accepts the existence of a clear moral authority and just chooses to disobey. The atheist has no guide to correct behavior except, perhaps, an intuitive understanding of what God ordains, and that is just not enough. The atheist faces no threat from disobedience other than social condemnation.
Now let me explain my perspective on morality as an atheist. The threat I face for immoral conduct is not just social, but we all face the penalty of social condemnation. I also face a personal psychological threat that is roughly the same as for the religionist. Moral rules are more like ethical rules in the sense that they are based on convention and principle. It is possible that my instinctive feelings of guilt, often based largely on empathy for others, were designed into me by a deity, but I really doubt that. More likely, they derive from the evolutionary process that created human beings as social animals. I want others to like me, and that is a powerful check on behavior. I also feel pressure to conform to social norms, even though I cannot always make sense of them on the basis of empathy or principle (e.g. "Do unto others...") I recognize instinctively that moral conduct makes me safer because it strengthens the social bonds that I depend on for comfort and survival. So there is a rational basis for moral behavior. Even though a god is not going to destroy me for misbehaving, I could lose standing in my community and self-esteem.
Now I will explain my problem with religious morality--why it concerns me that people ground morality in the authority of a deity. Gods can be capricious. They do not always have the best interests of humanity as a whole in mind. For example, some believers believe their deity wants their religious doctrine to be valued above survival and comfort. Sometimes religious law is harsh and cruel, but it is thought justified on the basis of how the deity feels about the behavior in question. That disturbs me because I regard gods (and supernaturalism in general) as grounded in pure imagination, not reality. Whether or not there is any truth to supernaturalism, it seems that people's beliefs about the supernatural can vary arbitrarily. So the moral grounding of a religious person has an element of arbitrariness that frightens me. Divine authority trumps all other authority, and it can contravene social welfare in general.
So we come full circle. I understand why people of religious faith question the basis of my morality and why they consider atheism a threat to social safety. I also see religion-based morality as a potential threat to human safety, although usually it is the case that people imagine their gods to want the same thing they do--safety and comfort for the human race.
On the other had, the "ceremonial deism" excuse strikes me as a transparent ruse to weaken the Constitutional requirement of a religion-neutral government. Deism is about belief in a God who doesn't intervene in human affairs. The term is misused here by theists, who feel that God will be more kindly disposed to us if we make every excuse to beg his help.
