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 is true of the Christian God – but also trivially true of any other conceptual deity which may exist. There have been thousands of God-figures discussed by mankind in its history, and if a god exists, none of those discussed may be the real God. (This strikes me as a kind of inverted version of the famous “Pascal’s Wager”)