Deism is a specific belief about God's non-interference, not just any spiritual belief. Although he's spiritual, he doesn't subscribe to deism's idea of a non-intervening creator.
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Historical Context
Deism was especially popular in the Enlightenment, when reason and science grew in importance. Many philosophers of the Enlightenment period advocated for deism over traditional religious belief.
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Not Atheism
Deism differs from atheism; it acknowledges a creator but denies divine intervention. He clarified that he's not an atheist; he believes in a creator, hence his interest in deism.