Jacob has given the allegory as an answer to the question of how the rejected Jesus can become the triumphant Messiah. He will now turn from the testimony of a past prophet to his own testimony of future prophecy.
Structural: Jacob says that he will prophesy, and he does. virtually his entire prophecy is included in the affirmation that the allegorical events "must surely come to pass."
For Jacob, prophesy is more than giving word of the future. In Jacob's use of the term, utterances under the power of his prophetic position are included under the umbrella of "prophesy," not because they foresee the future, but because they declare truth of God. This chapter will shift from this very succinct "prophecy" to admonitions to his people. Those admonitions will be the bulk of the text of the discourse, and fall under the rubric of true statements by a prophet - and therefore "prophecy" in this much wider definition.