The Lord perceives that one of Moroni’s concerns is that it is his weakness that will become the stumbling block of the future Gentiles who will not be able to accept the Book of Mormon. Moroni fears that his personal inadequacies will cause a lack of faith in some of the future generation. The Lord assures him that this is not the case.
[If they have not charity it mattereth not unto thee, thou hast been faithful]: The phrase “it mattereth not unto thee” might be better rendered “it is not your responsibility.” Of course it is legitimate that Moroni be concerned for others, but the Lord’s point is that agency is personal. When the Gentile with a lack of charity rejects the Book of Mormon, it will be because of that Gentile’s decision, not a fault of Moroni. The comfort to Moroni is in the Lord’s acceptance of Moroni’s efforts, for the Lord declares “thou hast been faithful.”
[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 ]: The Lord takes the theme of weakness and strength and applies it to Moroni. Moroni has understood his weakness, and has still tried to be faithful. He has been humble, and written when he did not think himself capable of power in writing. The Lord is accepting his sacrifice.
The “making strong” in this case skips over the natural consequences of the transformation of the soul and moves to the final glory of the soul that has been transformed into a celestial state. That person who endures to the end will receive the reward of the mansions. The Lord is telling Moroni that he has achieved the purpose of this life, to become a celestial person. That reward awaits him.