Moroni was comforted by the words of the Lord because all righteous men seeking to discern the mind and will of God have, at times, been troubled at heart to know His purposes concerning the Gentiles, or those who are in darkness because of unbelief. "O Lord, Thy righteous will be done," was Moroni's paean in praise of Him Who reigns On High. And inasmuch as the Lord then gave him assurance that the Gentiles would be shown their weakness and thereby made strong, he shouted praises because, "I know that Thou workest unto the children of men according to their faith." "Come unto Me, O ye Gentiles, and I will show unto you the greater things, the knowledge which is hid up because of unbelief." (Ether 4:13)
"And there was also written upon them a new writing, which was plain to be read, which did give us understanding concerning the ways of the Lord; and it was written and changed from time to time, according to the faith and diligence which we gave unto it. And thus we see that by small means the Lord can bring about great things." (I Nephi 16:29)
"And that He manifesteth Himself unto all those who believe in Him, by the power of the Holy Ghost; yea, unto every nation, kindred, tongue, and people, working mighty miracles, signs, and wonders, among the children of men according to their faith." (II Nephi 26:13)
"For behold, I am God; and I am a God of miracles; and I will show unto the world that I am the same yesterday, today, and forever; and I work not among the children of men save it be according to their faith." (II Nephi 27:23)
"Yea, and they did obey and observe to perform every word of command with exactness; yea, and even according to their faith it was done unto them" ... (Alma 57:21)
See also, Mormon 9:37.