[when ye shall see these sayings coming forth among you]: When the Book of Mormon is brought forth. “These sayings” is the text of the Book of Mormon.
[ye need not any longer spurn at the doings of the Lord]: This refers to those who would use the excuse that since the Second Coming is long delayed that it will never come. Mormon tells us that the very fact of the coming forth of the Book of Mormon will serve as a reminder that the Lord keeps his promises, and that the Second Coming will happen.
[for the sword of his justice is in his right hand; and behold, at that day, if ye shall spurn at his doings he will cause that it shall soon overtake you.]: This long phrase hinges on the “sword of justice” that represents the destruction of the wicked when the Triumphant Messiah comes. At that day when the Book of Mormon comes forth, it will be the sign that the last days are upon us, and we no longer may delay the consequences of our actions. If we “spurn his doings,” or reject the gospel, then justice will have claim upon us, and we will be among the wicked destroyed at the coming of the Triumphant Messiah.