It must have been a great shock to the mob in Ammonihah when one of their wealthy and formerly wicked fellow citizens stood up to defend Alma. Amulek launched into his talk by first reminding the people who he was.
Notice that Amulek specifically states that those who descended from Nephi are through the loins of Manasseh. He was the eldest son of Joseph. Having established his credentials by way of his illustrious lineage, Amulek now reminds the crowd of his long standing status as a citizen of the Ammonihah.