"And inasmuch as thou shalt keep my commandments, thous shalt be made a ruler and a teacher over they bretheren."
Nephi was appointed by God to hold the authority that would give him the right to lead his brothers to the promised land. Much like King David was appointed by a prophet to lead the people after Saul. Nephi proved his faith time and time again by following the commandments and always trusting in the Lord. Unlike David, though, Nephi didn't fall away as time progressed.
Authority from God give Nephi. Nephi have priviledge lead people and brothers promise land. Same David from Bible authority from prophet receive become king after Saul. Nephi faith always, follow commandments endure. Lord he trust. Different David, Nephi fall away, not.