If God, Jesus, the devil, angels, and demons were all real, we still couldn’t rely on the Christian Bible as gospal (pun intended) — it was written by humans, specifically men, who could have misunderstood or invented things. Think of it like the Snyderverse in Man of Steel: Superman is portrayed as a brooding, violent character — but that’s not his true nature. In Batman v Superman, Batman is straight-up killing people — also not his normal characterization.
So even if much of the Bible reflects real events or real beings, parts of it could still be distorted, exaggerated, or entirely made up by the people who wrote it down.
This doesn’t tell you anything if they were all derived from earlier stories – which, it turns out, is actually the case. We have the earlier stories as proof, in many cases.
There are centuries of religious thought by mystics developing upon the texts inspired in part by those stories. The parts based on common ancient legends comprise a relatively small part of religious texts.
And still, if anything that’s supportive evidence. The ancient legends that pop up again and again, that survive centuries of canonical revision, probably reflect deep and spiritually apparent features of reality.