Why do people think that religion is just about "being a good person"?

I often hear people say that they don't need religion to have to be a good person. True, religious faith does have a large moral component to it. But why do people who are not religious think that that's what it's all about? Why do they think that it's just about "don't steal, lie, or kill people"?

The definition of "religion", according to dictionary. com, is "a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."

The Modern Catholic Dictionary defines "religion" as "The moral virtue by which a person is disposed to render to God the worship and service he deserves. It is sometimes identified with the virtue of justice toward God, whose rights are rooted in his complete dominion over all creation. Religion is also a composite of all the virtues that arise from a human being's relationship to God as the author of his or her being, even as love is a cluster of all the virtues arising from human response to God as the destiny of his or her being. Religion thus corresponds to the practice of piety toward God as Creator of the universe."

So with that in mind, it would seem that the purpose of the practice of religion is more concerned with man's relationship with God than it is about simply "being a good person" (though again, that is an aspect of it).

What do you think?
Why do people think that religion is just about being a good person?
Why do people think that religion is just about "being a good person"?
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