The definition has been expanded (by whom and how I'm not sure) to include anyone of the Jewish faith.
‘Jew’ and ‘Jewish’ aren’t official terms. The technical term is ‘ben/bat yisrael’ (son/daughter of Israel).
The Bible relates that Mordechai of the Tribe of Benjamin was called ‘ish Yehudi’ (Jewish man), so the term apparently goes back to the Kingdom of Judah which included the Tribe of Levi and remnants of the other tribes.
The definition has expanded, I guess we should say Hebrew for clarification purposes.
Isaac was a Hebrew. Israelites are descendents of Jacob, so Jacob’s first son Reuben was probably the first Israelite.
“Jew” used to just mean descendants of Judah, or people from the Kingdom of Judah, but since the remainder of the other 10 tribes mixed in with Judah, you can probably say the first Israelite was also the first “Jew” as we use the term today.