Why did Paul refer to himself as a Jew, when he was actually from the tribe of Benjamin?
The word Jew came to be applied in two ways. It refers to any descendant of the literal tribe of Judah. Jew is a nickname; it is merely a shortened form of Judah.
But, Jew also referred to anyone who refused to join the rebellion at the time the twelve tribes split into two separate
kingdoms. As it turned out, all of the tribes with the exception of Judah, Levi, and Benjamin — formed the northern kingdom under the rule of Jeroboam. Their national name became Israel. The three remaining tribes, under King Rehoboam, formed the southern kingdom, called Judah. You may read of Benjamin’s alignment with Judah in I Kings 12:21.
Paul was a descendant of Benjamin. Nationally, however, he was a citizen of the kingdom or house of Judah. Thus, he was both a Jew and a Benjamite.
Of course, we also have to consider this question spiritually, as all converted Christians are called spirituals Jews.