He said he was the light of the world but that those who follow Him as he walked and talked were to become the light of the world. But as you can see nobody has really done that except me, at least from what I have encountered. He wanted those who follow him to deliver the same message as he did.
JN 8:12 "I am the light of the world, no one who follows me shall ever walk in darkness because he will possess the light of life."
Matt 5:1 When Jesus saw the crowds, he went up the mountain; and after he sat down, his disciples came to him. 2 Then he began to speak, and taught them, saying: 3 "Blessed are the poor in spirit, for theirs is the kingdom of heaven. 4 "Blessed are those who mourn, for they will be comforted. 5 "Blessed are the meek, for they will inherit the land. 6 "Blessed are those who hunger and thirst for righteousness, for they will be filled. 7 "Blessed are the merciful, for they will receive mercy. 8 "Blessed are the pure in heart, for they will see God. 9 "Blessed are the peacemakers, for they will be called children of God. 10 "Blessed are those who are persecuted for righteousness' sake, for theirs is the kingdom of heaven. 11 "Blessed are you when people revile you and persecute you and utter all kinds of evil against you falsely on my account. 12 Rejoice and be glad, for your reward is great in heaven, for in the same way they persecuted the PROPHETS who were before you. 13 "You are the salt of the earth; but if salt has lost its taste, how can its saltiness be restored? It is no longer good for anything, but is thrown out and trampled under foot. 14 "You are the light of the world. A city built on a hill cannot be hid. 15 No one after lighting a lamp puts it under the bushel basket, but on the lampstand, and it gives light to all in the house. 16 In the same way, your light should shine before others, so that they may see your good works and give glory to your Father in heaven."
The son of man has said He is the truth which is the word of God. He also said He is the Light. So if He is the Light then the light is also the word of God. And if your light must shine before others then the word of God must come out of your mouth which makes you a prophet. That is why He said they persecuted the prophets before you, in effect calling you a prophet. Not only that but He said your light must shine so they will see your good works, in effect saying that spreading the word of the Lord is good works.