Although Mormon “said I would speak” of “that faith,” that promise to cover the topic of faith is not contained in the discourse itself and therefore must have been made in some context outside of the discourse. Perhaps that context was the particular question that brought his congregation together in the first place, but the record does not explain the reference.
Mormon promises that his listeners may “lay hold on every good thing” through the operation of faith. Key to his interpretation is the concept he has already established: that good things are those that lead toward God.