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What book did James I write to support his belief in witchcraft?
King James I did not write any books of the Bible. Instead, he had paid scribes. He had words such as "poisoner" and "wickedness" changed to "witch" and "sorcery" to support his aversion toward the local poor Pagan communities.
Just the Epistle of James. It is not certain which James wrote it. It was probably 'James the brother of the Lord', who was neither of the apostles, James son of Zebedee or James son of Alpheus.