Literary: It is mildly unfortunate that verse 16 has been separated from 15, as the last phrase of 15 is the first part of a parallel that is ended in 16. The text should read:
"Thou art blessed because of thy exceeding faith in the words alone of my servant Abinadi.
And blessed are they because of their exceeding faith in the words alone which thou hast spoken unto them."
The Lord is noting blessings for faith. In particular, the Lord emphasizes faith in the words of another. The first example is Alma's faith in the words of Abinadi. The second example moves Alma into Abinadi's position, and has people believing upon the words of Alma. The parallel phrases emphasize the overall statement that man may be blessed if he listens to the words of those who speak for God. We do not need that experience ourselves, but may gain the full benefits through faith upon the words of others.