Moroni expands on the Savior’s mission. He has fulfilled the Mosaic law, and therefore it may be done away with. Nevertheless, this shift from the law of Moses to the Messiah’s gospel does not automatically redeem us. There is a difference between the Savior’s universal sacrifice, which redeems us from death, and our agency in accepting that sacrifice, which redeems us from sin. By saying, “he claimeth all those who have faith in him,” Mormon testifies that the atonement happened, but despite its sureness, it becomes operational only on condition of our acceptance. It absolutely happened, but we must absolutely accept it, or it has no effect on us.
Here Mormon begins to tie everything together, explaining that “they who have faith in him will cleave unto every good thing.” This is his definition of faith. Obviously, those who truly follow the Savior make the commitment to live the gospel; that (by definition) is the way to lay hold on all good things.
Verse 30 should be read with verse 29, since “they” refers to the angels (v. 29) whose ministrations are a type of miracle that still occurs for those “of strong faith and firm mind.”