Is the Old Testament a Christian Book?

Yesterday I started describing a trade book that I’m thinking about writing, tentatively called (in my head) “The Battle for the Bible.”    Here is the next part of my self-reflections:

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A major part of my book will deal with one of the great puzzles in the history of religion:  Why does the Christian Bible even have an Old Testament?   And how did the early Christians, most of them gentiles, manage – in their own minds — to wrest it from the Jews b...