What makes Good News good? Part four

Continuing thoughts on John Piper's sermon 'What makes the Good News good?'
God uses people to bring His light to others.
Acts 26:17-18

At the end of the last section we were left with an apparent contradiction. If God alone can take away the darkness and shine light, what is our role in evangelism? Why are we told to Go! at the end of Matthew? In Acts 16:17-18 Paul says that he has been sent to the gentiles to open their eyes so that they may turn from darkness to light, from the power of Satan to God. So it seems that God uses human messengers to do what only He can do.
I have never come away from sharing the gospel with someone feeling like a did a good job. I've always felt like i've put my foot in it somehow, or said the wrong thing, or not enough, or too much. I feel helpless. And i am. But God has appointed us as agents of His message, so we must therefore go with confidence that every authority on Heaven and Earth has been given to Him. Every authority. So we have nothing to fear. This is why 2 Corinthians 4:7 comes right after 4:6. Because its true. We are weak, we are, on our own, as useless as a broken clay pot. And yet, inside us we have an uncountable and insurmountable treasure. The light of the knowledge of God in the face of Jesus Christ. The sweetest and best light there is.
Yes, only God can replace darkness with light. But yes, we have a responsibility to be His messengers, to go and tell people. This is why we are like clay pots. Because God wants all the glory due His name. God makes it so that we have to rely on Him in evangelism, in the business of helping people see His light because that's how it should be. Only God has the power to take away the darkness, and so He should be treated and revered as such. God wants the glory. Acts 26:17-18 tell us that we must be part of this, that we have to tell people, that we have a part to play in opening people's eyes. And what an honour it is to play that part, to be with people as their eyes open for the first time.
So what role do we have to play as messengers? That question is what Piper deals with next.