John 8: 12-18, The Light of the World

Jesus has been preaching in Jerusalem, in the temple courts.

John 8: 12-18, Light of the world
When Jesus spoke again to the people, he said, “I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life.”

13 The Pharisees challenged him, “Here you are, appearing as your own witness; your testimony is not valid.”

14 Jesus answered, “Even if I testify on my own behalf, my testimony is valid, for I know where I came from and where I am going. But you have no idea where I come from or where I am going. 
15 You judge by human standards; I pass judgment on no one. 
16 But if I do judge, my decisions are true, because I am not alone. I stand with the Father, who sent me. 
17 In your own Law it is written that the testimony of two witnesses is true. 
18 I am one who testifies for myself; my other witness is the Father, who sent me.”

In these dramatic statements, Jesus offers special light, eternal light, out of human darkness.  Those who follow him leave that darkness, living in light. In the other gospels, Jesus invites people to join the kingdom of God.  The invitation here is similar... and different.

Of course, the religious leaders challenge him, for they claim to be represent Judaism. Jesus responds that his words are valid, that he has a special calling, a special path and relationship with God. So he has made a claim as a witness, but God does also.

In verse 15, Jesus accuses the Pharisees of judging others, and doing it poorly.  He, on the other hand, does not judge (not at this time) but offers light and healing.

I don't really understand the "two testimony" argument. One could argue that God has already testified for Jesus in Old Testament prophesy and in some of Jesus's miracles.  But Jesus also insists on testifying publicly, clearly, as to who he is.