[of which I said I would speak]: There is no antecedent to this phrase. Mormon is stating that he has some kind of promise that this discourse would cover the topic of faith, but that promise is not contained in the discourse itself. We can only assume that this promise was made in some context outside of the discourse. Perhaps the promise of the topic was the particular reason for the gathering of the congregation that heard this discourse originally. We do not know.
[now I come to that faith… and I will tell you the way whereby ye may lay hold on every good thing]: Mormon is making an interesting connection here. He is promising that we may “lay hold on every good thing,” and that it is faith that will allow us to do this. Key to his interpretation will be the understanding of the good things as those things that lead towards God.