Why did Nephi and his brothers return to Jerusalem from the Red Sea (the first time)?
To retrieve their father's (Lehi's) brass plates.
And it came to pass that I, Nephi, returned from speaking with the Lord, to the tent of my father.
And it came to pass that he spake unto me, saying: Behold I have dreamed a dream, in the which the Lord hath commanded
me that thou and thy brethren shall return to Jerusalem.
For behold, Laban hath the record of the Jews and also a genealogy of my forefathers, and they are engraven upon plates of brass.
Wherefore, the Lord hath commanded me that thou and thy brothers should go unto the house of Laban, and seek the records,
and bring them down hither into the wilderness.
1 Nephi 3:1And it came to pass that I, Nephi, returned from speaking with the Lord, to the tent of my father.
And it came to pass that he spake unto me, saying: Behold I have dreamed a dream, in the which the Lord hath commanded
me that thou and thy brethren shall return to Jerusalem.
For behold, Laban hath the record of the Jews and also a genealogy of my forefathers, and they are engraven upon plates of brass.
Wherefore, the Lord hath commanded me that thou and thy brothers should go unto the house of Laban, and seek the records,
and bring them down hither into the wilderness.
1 Nephi 3:1