Nephi told Jacob to record the most important preaching and revelation. Jacob notes that they did receive revelations. For Jacob, the most important revelation was the same as it was for Nephi. They “knew of Christ and his kingdom, which should come.” Jacob does not give any new information about how those revelations might have differed from what Nephi had written. It sufficed that they knew of this Messiah who was to come. Phrases discussing what was to come in the Book of Mormon typically point to the mortal mission of the Messiah.
Jacob declares that he taught as did Nephi. The preaching was to bring his people to the understanding of the atoning Messiah. As with Nephi, we may expect that Jacob also taught that the law of Moses was to be lived while understanding the future atonement and its present meaning.
Having stated the commission Nephi gave him concerning the plates, Jacob declares that he did “take it upon me to fulfil the commandment of my brother Nephi.”