Could someone show me where even the first known instance or occurrence of the word "Yahadut" even is? Quite a while back I asked a previous Hebrew teacher for the etymology or origin & she asked to get back to me on that, but she never did, I think … so … since when is even this word even around? I understand after the destruction of the Second Temple people re-invented stuff since per Daniel 9 & Haggai 2 timelines the Messiah Prince (Maschiach Nagid) had to come at the time of the Second Temple …
Mordechai was referred to as "hayehudi". In Talmud the religion is called "דת יהודית". Don't know specifically about "yahadut".
Originally, "Yehudi" meant "from the tribe of Yehuda", or from the kingdom of Yehuda, as opposed to the biblical kingdom of Israel. So Mordecai HaYehudi likely belonged to the tribe.