[And I said unto him]: In the previous verse Moroni refers to a commandment he has received from Jesus Christ concerning his role in fulfilling the promise to bring the Book of Mormon to the world.
[the Gentiles will mock at these things, because of our weakness in writing] Moroni either understands very well, or has seen a vision of the future reception of the Book of Mormon. While it is intended equally for the Lamanites, his concern is for the derision with which it will be held in the eyes of many of the Gentiles. Moroni suggests that they will deride the work because of the “weakness in writing.”