Judah was the southern kingdom of Israel, formed after the division of the united monarchy during the reign of Rehoboam, made up of the tribes of Judah and Benjamin with Jerusalem as its capital. It was the homeland of Lehi and his family before their departure for the promised land. In the first year of the reign of Zedekiah, king of Judah, prophets warned the people of Jerusalem to repent or see the city destroyed (1 Nephi 1:4), and the record on the brass plates ran down to the start of that reign (1 Nephi 5:12). Zedekiah was carried captive into Babylon, and the people of Zarahemla left Jerusalem at that time (Omni 1:15).
The Isaiah chapters reproduced by Nephi address Judah directly. They name its struggles with neighboring kingdoms — the plan of Syria and Israel to go up against Judah and set their own king over it (2 Nephi 17:6) — and foretell that the adversaries of Judah will be cut off and the rivalry between Ephraim and Judah end (2 Nephi 21:13). Isaiah also declares that Jerusalem is ruined and Judah is fallen because their words and deeds were against the Lord (2 Nephi 13:8).
Judah figures in the symbolism Nephi draws from these texts. The house of Israel is described as coming “forth out of the waters of Judah, or out of the waters of baptism” (1 Nephi 20:1). And the writings of the loins of Judah and the writings of the loins of Joseph are to grow together, confounding false doctrine, ending contention, and establishing peace (2 Nephi 3:12).