The patriach of the Hebrews was Abraham, who was the father of Isaac, who was the father of Israel, who was born with the name of Jacob.
Israel believed in the God of his father and his grandfather, the God that told Abraham that there was just one true God and that he must left his home to go to the promised land. So, the answer is: TRUE, the first Israelites were monotheistic, since they were the descendants of Israel/Jacob and believed in the God of their ascestors.
