Alma 46:21

And it came to pass that when Moroni had proclaimed these words, behold, the people came running together, with their armours girded about their loins, rending their garments in token, or as a covenant, that they would not forsake the Lord their God; or, in other words, if they should transgress the commandments of God, or fall into transgression, and be ashamed to take upon them the name of Christ, the Lord should rend them even as they had rent their garments.

When Moroni said these words, the people came running together with their armor on. They tore their clothes as a sign or promise that they would not leave the Lord their God. In other words, if they did wrong and broke God’s commandments, or if they became ashamed to take the name of Christ, then the Lord would tear them like they had torn their clothes.

When Moroni made this declaration, people came running with their armor on, tearing their own clothes as a sign—as a covenant—that they would never forsake God. They were saying that if they ever broke God’s commandments or turned away in shame from the name of Christ, God could tear them apart just like they’d torn their clothes.