According to original research by John Sawyer and John Welch, the term mosiah was an ancient Hebrew term, like go'el ("redeemer, or avenger of blood"), or sedeq ("victor, savior"). Such terms originally had meaning in Hebrew daily life and culture but came to be used among their titles for God. The word mosia (pronounced moe-shee-ah) is a word peculiar to Hebrew, a "word invariably implying a champion of justice in a situation of controversy, battle or oppression."
Apparently the form of the word Mosiah is a "hiphil participle" in Hebrew. It occurs in the Hebrew in Deuteronomy 22:27, 28:29; Judges 12:3; Psalms 18:41; and Isaiah 5:29--texts that in all probability were on the Plates of Brass. This word, however, was not transliterated into the English by the King James translators, and thus the Hebrew would not have been known to Joseph Smith. It was, however, known and used as a personal name in the Book of Mormon, as well as by people in the Jewish colony at Elephantine in the fifth century B.C. . . .
King "Mosiah"(1) (Omni 1:12) was a God-appointed hero who delivered the chosen people of Nephi from serious wars and contentions by leading them in an escape from the land of Nephi (see Omni 1:12-14). It is unknown whether he was called
“Mosiah Who Was Made King Over the Land of Zarahemla”
When was Mosiah1 born and when did he depart for the land of Zarahemla? We can approximate those years by tracing backward from the dated death of Mosiah2 to Mosiah1:
Year (Years Since Lehi Left Jerusalem)
510 Mosiah2 dies in the 33rd year of his reign (Mosiah 29:46).
Mosiah2 is about 63 years old at his death (64th year of his age) (Mosiah 29:46).
It is in the whole 509 years from when Lehi left Jerusalem (Year 510) (Mosiah 29:46).
477 This was the first year of Mosiah2's reign.
446-7 Mosiah2 was born.
Let's allow for an average age of the father king at the time of birth of his successor son to be within the age period of from 30-40 years old. We will first assume that Benjamin was 40 years old when Mosiah2 was born and had been reigning since about age 30. According to Mosiah 6:4,
Mosiah2 began to reign "in the thirtieth year of his age" (age 29-30). Thus:
407 Benjamin was born.
We will next assume Mosiah1 was near 40 years old when Benjamin was born and had been reigning for 10 years since his thirtieth year (for ease of calculation we will say age 30). Then:
367 Mosiah1 was born close to the year 367 [40-year option].
If both fathers (Benjamin and Mosiah1) had been 30 at the birth of their heirs, then Mosiah1 would have been born in the year 387. Thus:
Year 367-387 The range of years for the birth of Mosiah1.
If we use Mosiah2's age (29-30) at his ascension as a standard for every king (Mosiah 6:4), Mosiah1 would have been close to 30 and ascended to the throne between the years 397-417. If we allow a 30 year time period before the end of that reign, then whatever the age of Mosiah1 when he left the land of Nephi for Zarahemla, that departure would have been sometime between the years 397 and 447 (since Lehi left Jerusalem). Because an older less vigorous man might not be accepted by the people of Zarahemla as "king Mosiah," I would tend to put the departure of Mosiah1 towards the beginning of the period of years 397-447 (since Lehi left Jerusalem).
Year 397-447 Mosiah1 departs for the land of Zarahemla.
[Alan C. Miner, Personal Notes] [See Appendix A]