Jacob knows that he has been hard on his congregation. While he has promised salvation, he has just specifically noted ways in which they might be condemned. His reminders in the series of woe-statements were probably references to things that were actually happening in the community. This was not a hypothetical sermon, but one designed to alter behaviors. We should remember that Nephi the king suggested that Jacob speak on this topic. It was not a question of theology that concerned Nephi, nor Jacob.
Jacob declares that the way out of the conditions of the woe-statements is to follow the ways of the Holy One. Jacob’s language about the way of the Lord ultimately comes from a reference to Isaiah 40:3–4, “The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God. Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain.” The specific language, however, borrows from Matthew 7:11, “Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.”
It is not uncommon in the Book of Mormon’s translation to see places where there is a reference to a scripture from the plates of brass that is expressed in the way the New Testament couches the reference.
The concept of the straight course conveys that there is a clear way to follow God. God has given his law, and if we follow that law, we will ultimately have eternal life. The narrow gate suggests that it is only through God’s law that this may occur. There may be other available paths, but God’s law will winnow out the unrighteous who cannot enter at the narrow gate of His law.