Jacob's first recorded sermon identifies what concerns him. It is the "welfare of souls" (2 Ne. 6: 3) and "things which are, and which are to come" (2 Ne. 6: 4.) The definition of truth is knowledge of things which are, which were, and which are to come. (D&C 93: 24.) Jacob is interested in teaching truth. But the truth he wants to focus on is the present and future of his people.
He identifies Isaiah as speaking "concerning all the house of Israel" (2 Ne. 6: 5) and therefore they can be likened to the Nephites. Then he turns to the Gentiles and places them in the future role of "bringing thy sons in their arms, and thy daughters shall be carried upon their shoulders." (2 Ne. 6: 6.) In the dismal future of Nephite destruction by the Gentiles, there is still a more distant day when Gentile efforts will become helpful, not destructive. When that happens, the Gentile fortunes are reversed, and they will "bow down to [the Nephite remnant] with their faces towards the earth, and lick up the dust of [Nephite] feet." (2 Ne. 6: 7.) So the cataclysm which befalls the Nephites will also befall their Gentile vanquishers. They will be brought down to the dust as well.
Jacob also reports to his audience "the Lord has shown unto me that those who were at Jerusalem, from whence we came, have been slain and carried away captive." (2 Ne. 6: 8.) Jacob must have asked to be shown. He asked and was shown, and therefore he knew his family had left Jerusalem in time to avert death or captivity. Jacob was born after they left Jerusalem; but he knew about it, inquired to know, and was shown their destruction.
This reaffirms how the departure by Lehi and the destruction of Jerusalem was inter-related. The Lord uses 'just-in-time' scheduling of events more often than not. There is no need to flee until the moment when the destruction is about to begin. Nor is there a need to begin the rainfall before the ark is completed. Nor is there a need to send down fire to consume the offering until the altar is built, the sacrifice offered, the water poured on the offering, and the prayer completed. (1 Kings 18: 31-38.) Timing is always the Lord's.
Jacob also leaves nothing to the imagination of his audience. He tells them the Messiah will come to Jerusalem, will be scourged there, and will be crucified by them. Jacob knows this "according to the words of the angel who spake it unto me." (2 Ne. 6: 9.) From this we see Jacob's pre-sermon preparation does not consist of gathering together thoughts and quotes from poets or philosophers. He consults with angels and dispenses information from heaven. Here is a source which is to be trusted. When speaking of Jerusalem's destruction, it comes from the Lord's showing him, and of the Messiah's mission. It comes from the angel's speaking to him.
We think it an odd thing to have a man speak with the Lord and be ministered to by angels. Yet in the example of Jacob, it is almost matter-of-fact. As if he wouldn't dream of speaking about such things without consulting with heaven.
Nephi's brother Jacob is among the great figures in all of sacred scripture. The critical differences between him and his teaching, and other men giving what they regard as inspirational thought, should not pass by unnoticed. I'm growing to respect this man Jacob.
