In the fulness of time the Christian religion sprang out of Judaism; as a fact, of divine revelation, but also inseparably joined by innumerable threads with the previous thousand years of Israel's history. No incident in the gospel story, no word in the preaching of Jesus Christ, is intelligible apart from its setting in Jewish history, and without a clear understanding of that world of thought- distinction of the Jewish people.
Thus it becomes the bounden duty of Christian theologians to examine into and describe that realm of thought and history in which the universal religion of Christ grew up. Nor is it enough to know simply that older literature which has been collected together in the canon of the Old Testament. On the contrary, the gospel of Jesus Christ is much more closely connected with its immediately contemporary surroundings, and the tendencies of thought prevailing in that particular age.
from page 1, Volume I, History of the Jewish People