Jacob supplements his scriptural testimony with his personal testimony. Jacob places his authority in both the scriptures and the ultimate source of the scriptures, in God himself. Jacob has now placed a dual pressure on Sherem, he has opened the scriptures and presumably provided their Messianic interpretation, and he has then increased the pressure on Sherem by asserting his own prophetic mantle.
As an outsider to the town of Nephi, Sherem did not know Jacob, and while it was probably his mission to discredit Jacob, his knowledge would have extended only as far as his secular assumptions would have taken him. Sherem would have expected a sage - but not a prophet.