[If so, his faith and hope is vain, for none is acceptable before God, save the meek and lowly in heart]: This is not a new qualification in Mormon’s discussion. Verses 5-13 discussed the relationship of the actions to the motivation behind the actions. This is essential to his current argument. To lay hold of all good things, one must have faith, which involves actions. Those actions can only be good if our intentions are good, and that comes from being humble before the Lord so that our actions are in accord with the path he has laid out for us.
[for if he have not charity he is nothing]: The next move in this careful explanation is to link faith and hope to charity. Mormon has done this by moving from the external action of faith to the internal motivation of hope. Now that he is dealing with the heart of the person, and the need for humility, he can link charity to those concepts as the external evidence of the internal humility. Thus the progression of concepts moves from external (faith manifest in the actions we take), to the internal (the hope that motivates us to a specific type of faith), and finally a return to the external (in charity – the manifest external actions of one who is internally humble before God.) This is the reason that we are “nothing” without charity. This is the witness of the change in our hearts that is part of the path towards becoming like God. If we have not achieved this love of our fellow man (in at least some degree) we are not on the correct path, and “every good thing” will remain distant from our grasp because we are not doing that which is required to achieve it.
Reference: The idea that one is nothing without charity echoes verse 2 of 1 Corinthians 13:
1 Corinthians 13:2
2 And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing.