God existed at the time the Old Testament was written, but as he was exactly as described in the book, theists had to reject him as Jews had rejected the gods of their fathers and the Jews’ fathers, as well as the fathers of the Greeks, the Egyptians and the Sumerians, had rejected the gods of their own forefathers.
Today’s god (theologians do not believe in Christ), the out-of-the-universe one, is not an entity, it is a big fat onion consisting of layers of rejections.