The question isn't whether the Jews were monotheistic long before Jesus's time, but likely when this clarity occurred.   

The latest I've seen is somewhere between 700-800 BC.  That's long enough for God to have worked this clearer understanding of God into the Israelite people to serve God's plan of Salvation especially through His Son Jesus, a Jew.

Jews clearly in that time well understood the meaning of: Shema Yisrael, Adonai Elohenu, Adonai Echad.