There are two sources, or starting points, for the idea that a god exist (the god idea).

1. Religious scripture or oral tradition that tells a story about a god, and claims the described god is the divine creator (supreme creator). The Bible is one source, but there's nothing in these stories that could not have been invented by people. Revelation by an actual god was not necessary. There is no specific detailed information in any of these texts that only a god could have known.

2. The belief that there is some form of intelligence behind the energy and matter that started the universe. This is the first cause question. If the universe must have had a creator, then what created the creator? This is a mystery that cannot be explained by assuming an intelligent creator which would be an even bigger mystery. If we assume a creator always existed, then why not assume the universe, in some form, has always existed.

A more elaborate answer

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