Moroni expands just a bit on the mission of the Savior in that he has fulfilled law, and therefore the law of Moses may be done away. Nevertheless, this shift from the law of Moses to the gospel of Christ does not automatically redeem us. There is a difference between Christ’s universal sacrifice, and our agency to accept that sacrifice. This is the intent of the phrase: “he claimeth all those who have faith in him.” The Atonement happened, but it is conditional even if it is sure. It absolutely happened, but we must absolutely accept it, or it is of no effect on us.
[they who have faith in him will cleave unto every good thing]: Mormon begins to tie everything together. Those who have faith in Christ are those who will “cleave unto every good thing.” He uses this as a definition, without explaining why it is so. The assumption that is made is that those who truly follow Christ make the commitment to live the gospel, and that, by definition, is the way that one may lay hold of all good things.