In Chapter 1 the author wants to ascertain what an Old Testament prophet was not. Scripture informs us of prohibitions on the function of a prophet. The author explores these prohibitions first in Deuteronomy. He considers Deuteronomy 13:1-5 and Deuteronomy 18:9-13.
In Deuteronomy 18, God taught His people to live under the revelation of His word as mediated by Moses. Deuteronomy 18 finds its fulfillment in Christ as the Prophet who not only mediates God's revelation, but is Himself that revelation.