If we did not yet know that this was written after the fact, it is clear from Jacob’s statement in verse 8. Jacob declares that he confounded Sherem. Then he relates the exchange that demonstrates how he did it. The exchange is related in terms of a dialogue.
Sherem wanted to preach against the coming atoning Messiah, so Jacob begins with that topic. Jacob asks if Sherem denies the Messiah who is to come. Sherem’s response is that he cannot deny something that does not exist. The basic argument is that this Messiah does not exist in the present, and did not exist in the past. Neither of those are points that Sherem can argue. Sherem does leave the door open, however, because he also says that he does not believe that there will ever be such a Messiah.
Jacob cannot turn to history, but he can turn to prophets and prophecy. They are found in the scriptures, so Jacob makes certain that Sherem indicates that he believes in the scriptures. Sherem says yes, and Jacob tells him that he must, therefore, not understand them, as they speak of this coming Messiah, and Jacob himself is a prophetic witness. Again, this is not any Messiah, but the atoning Messiah, as Jacob makes clear in verse 12.
Thus, Jacob has declared, but only asserted, that the scriptures point to the atoning Messiah. That might be a difficult thing to prove as Nephi apparently believed that much of the teaching about that coming atoning Messiah had been removed, or left out of, the scriptures.