Once again we have Jacob speaking in the plural. Particularly on the basis of the evidence of this verse (with the power of the faith of the "we") we must see "we" referring to Nephi and Jacob.
Literary: Jacob earlier noted that all of the prophets had testified to the mission of the Savior. Here is uses that fact as the foundation for the reason he and Nephi searched the scriptures. After their searches, Nephi and Jacob also have their own revelation as witnesses. All of their spiritual understanding led them to their firm faith, a faith so powerful that it could command even nature.