In Alma 46:12 it says the Moroni rent his "coat." According to Ixtlilxochitl, the Tultec men of Mesoamerica wore mantles. A mantle is a long cloak or robe, short-sleeved or sleeveless. This was the outer garb of the ancients of Palestine also. Sometimes the outer garment worn by men was referred to as a cloak and sometimes as a robe. In either case it constituted a mantle.
The undergarment of those of ancient Israel, both in the Near East and Middle America, was a simple tunic, also known as a shirt or coat. It was a sleeveless garment reaching down to the knees, like a long sleeveless shirt . . . [Milton R. Hunter and Thomas Stuart Ferguson, Ancient America and the Book of Mormon, p. 317]
“He Rent His Coat”
It is interesting that when faced with a kingship rebellion led by Amalickiah (see Alma 46:4-7), Moroni not only took it as an attempt to replace Christ as the true head of the Nephite nation (see Alma 46:8-16). In his sorrow, he tried to warn the people of how their status could be lost if they were to allow this to happen. He emphasized his point by rending his clothing (Alma 46:12).
According to Donna Nielsen, this scenario alludes to some other critical times in Jewish history. She writes that one Hebrew translation of Matthew 27:51 says that at the death of Christ, " . . . the curtain of the temple was torn in two from top to bottom." Apparently in this one act, Heavenly Father did a very Jewish thing, but because Judaism has been downplayed in Christian understanding throughout the centuries, one can easily miss the significance of this event. In addition to the spiritual meaning spoken of in the ninth and tenth chapters of Hebrews, there is another literal understanding that is also valid. Avi Ben Mordechai sheds more light on this subject.
I will let the ancient scribe of Second Samuel tell you its literal meaning:
While they were on their way, the report [rumor] came to David: "Absalom has struck down all the king's sons; not one of them is left." The king stood up, tore his clothes, and lay down on the ground; and all his servants stood by with their clothes torn (2 Samuel 13:30-31).
Here at the death of Y'shua, is a classic example of divine Hebraic mourning. The Temple was the house of God. The parakhet or curtain (the partition separating the Holy of Holies from the Holy Place) was traditionally referred to as the "hem of His garment." The curtain ripping would have been understood by the priests as symbolic of God rending His garment. Remember there were tens of thousands of Jews gathered for Pesach (Passover) and Unleavened Bread. When news of the ripped parakhet passed from the priests to the people, I think it became a testimony to them. No doubt, many understood at that point Y'shua was Israel's Anointed One and that [God] was grieving over the death of His Son."
[Donna B. Nielsen, Beloved Bridegroom: Finding Christ in Ancient Jewish Marriage and Family Customs, p. 142]