As already noted (Ether 12:23), Moroni fears that his personal inadequacies will be a stumbling block to faith for the Gentiles. Yahweh assures him that this is not the case. Their lack of charity “mattereth not unto thee” because “thou hast been faithful.” By “mattereth not,” Yahweh means that it is not Moroni’s responsibility because he has faithfully discharged his duty. Of course, concern for others is legitimate; but Yahweh’s point is that charity-lacking Gentiles who reject the Book of Mormon will be making their own decision. The Lord comforts Moroni: “Thy garments shall be made clean.”
The Lord applies the theme of weakness/strength to Moroni. “Because thou hast seen thy weakness thou shalt be made strong, even unto the sitting down in the place which I have prepared in the mansions of my Father.” Moroni has understood his weakness, humbly continued when he did not think himself capable of achieving powerful writing, and has made an acceptable sacrifice.
“Making strong” in this case skips over the process of soul-transformation and moves directly to the celestial glory of the transformed soul. The person who endures to the end will inhabit “the mansions of my Father.” Yahweh is assuring Moroni that he has become a celestial person.