"And it came to pass that he departed into the wilderness. And he left his house, and the land of his inheritance, and his gold, and his silver, and his precious things, and took nothing with him, save it were his family, and provisions, and tents, and departed into the wilderness."
Lehi proved his loyalty to the Lord by willingly leaving all thins of worldly value and taking only the things they would need to survive in the wilderness. Every prophet and disciple of Christ has sacrificed something for the truth of the gospel. The testimony of the prophets is solidified and set by the sacrifice of the prophets.
From the time Lehi leaves Jerusalem the records of these people is started. Their timing is compared to the time Lehi left Jerusalem, which equals to 600 years before Christ's birth to give a reference point.