Deuteronomy 18:19
And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him. And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
The prophet like Moses promised in the verse is Christ. That Prophet speaks God's words in God's name. His authority and power are supreme. His coming is the distinguished arrival of the heaven-appointed Teacher, whose words must not be disregarded.
Whoever will not hearken unto the words he speaks in God's name will be held accountable to God, who will require it of him. Refusing to heed him brings destruction, and the warning applies to those who reject his message.
Believing Moses' writings therefore means believing in Christ, whom Moses foretold and whose testimony Jesus applies to himself. The Pharisees trusted Moses' writings yet rejected Christ, whom Moses had written about, and their refusal to believe Christ's words exposed their failure to believe Moses.