The distinction between fiction and literal truth is an anachronism, since it depends on the scientific worldview in which there can be such a thing as a purely objective fact.
The fraud would be holding out the myths and scriptures as mere literal or historical truths, and that fraud is either a fundamentalist anachronism or, in the specific case of Christianity, a Catholic betrayal and compromise with the Roman Empire.
If you read scriptures in the premodern way that taps into their authentic potential, you’re reading them as fictions (that can contain psychological or subjective truth), not as pseudoscientific life manuals or historical textbooks. If you read them as organized religious institutions tell you to read them, you’re reading them as literal revelations from a personal creator of the universe, which would be preposterous.
I note and am amused by the veiled threat of hellfire in your parting bit of childlike foolishness, “but what you believe is your responsibility.” Here you fallaciously compare belief with an act for which you can justly be held responsible, such as the decision whether to give to charity or to rob a bank.
Do you choose to believe the sky is blue? Do you choose to think two plus two equal four? Do you decide to believe that the United States is part of North America? How we understand the world isn’t made up of free choices for which we can be held morally responsible.
Atheism, then, is obviously not a sin. I could no more choose to change my beliefs about religion than you could just decide to believe that a mouse is larger than an elephant. Just try to believe what you know is false and see how far you get. And what should we make of a deity who would punish you for having the wrong beliefs? The answer is that such a deity is a character in an ancient work of fiction.