Moroni was a man of great faith. Yet, when the city of Nephihah was lost, he began to doubt. We are reminded of the prayers of Mormon, who wrote, my soul had been poured out in prayer unto my God all the day long for them; nevertheless, it was without faith, because of the hardness of their hearts (Mormon 3:12). In both of these instances, faith was weakened by the wickedness of others. Great faith is that belief that anything is possible with the help of the Lord. Indeed, this is the promise of the Lord to us individually, all things are possible to him that believeth (Mark 9:23). When our individual abilities are inadequate, the Lord is possible to make up the difference in all situations.
However, when we are concerned for the welfare of others, our faith cannot be limitless. We can pray for the wicked to repent, but it won’t always happen. We can struggle to win the souls of men, but not every battle is won. This is because the Lord can control everything in the Universe, but He cannot interfere with man’s use of agency. When Moroni began to doubt and Mormon prayed without faith, it was because they understood that the Lord can control everything but man’s will—his inalienable right to do wrong.