In Genesis 10, why is the reference made to Gentiles when the Jewish faith had not yet been established?
First we must understand that what we call the Jewish faith originated in the Garden of Eden right after the fall of Adam and Eve. It was only being formalized during the time of Moses in the wilderness wandering.
That said, some English translations of Genesis 10:5 contain the word “Gentiles” along with the word “nations”. In Hebrew the two words are the same, so the distinction between “Gentiles” and “nations” only appears in the English language. In addition, the Book of Genesis was written as a history of God’s people by Moses several hundred years later, after the separation between Jews and Gentiles had become obvious following their Exodus from Egypt.