The “when” in “when ye shall see these sayings coming forth among you” means the coming forth of the Book of Mormon (“these sayings”).
Mormon directly challenges those who assert that the long-delayed second coming of Christ will never come: “Ye need not any longer spurn at the doings of the Lord.” The Book of Mormon’s coming forth is a reminder that Yahweh keeps his promises. Therefore, his second coming is sure.
Those who “spurn his doings” (reject the gospel) will be destroyed. Christ’s coming in triumph is symbolized by “the sword of his justice… in his right hand,” which will fall upon the wicked.
“These sayings” (the Book of Mormon) announces prophetic promises concerning the Triumphant Messiah and the last days. Upon those people who receive the promised witness (the Book of Mormon), but refuse to accept it, Mormon pronounces a series of prophetic laments.