I think "the" event that turned the Hebrews into monotheists was Abraham's encounter with God and God's messengers. Abraham and his wife, Sarah, were very old, both in their nineties, but God promised that they would have a son. Of course, they were skeptical; what woman in her nineties could get pregnant? Yet that is exactly what happened: Sarah got pregnant and gave birth to a son, whom they named Isaac, which means "laughter," because she had laughed when told she would have a baby.
When God made the promise that they would have a son who would be the founder of a great nation, Abraham gathered all his male servants, and they all were circumcised as a sign that they believed in and would now follow God. From that day, Abraham and everyone associated with him worshiped the one God.