Lehi had obtained a promise, a covenant, that went with the land. They would prosper if they lived according to the commandments. During Mormon’s time, they failed so utterly that they invoked the negative aspect of the covenant. If they did not follow the commandments, they would be destroyed.
Mormon declares that, even though the negative promise of destruction is being enacted upon the Nephites, God has not removed the covenant. The “Lord hath reserved their blessings.” Those blessings have been transferred to “the Gentiles who shall possess the land.”
That divine process is alluded to in Luke 19: 26: “For I say unto you, That unto every one which hath shall be given; and from him that hath not, even that he hath shall be taken away from him.”