St Peter is a figure of contradictions. Last week we heard him rightly acclaim Jesus to be 'the Christ, the Son of the living God'; we heard Jesus call Peter the 'rock' on which he was to build his Church; we heard Jesus promise Peter the keys of the kingdom with the power to bind and loose on earth and in heaven.
It was looking like Peter was starting to cotton on to who Jesus was and what he was about. That is, until we hear the continuation of the story this week, when Peter betrays his lack of understanding.
He doesn't understand that the kingdom of heaven to which he himself will be given the keys will only come to its fulfilment after the Christ has suffered, been killed, and been raised from the dead. Immediately, Peter shows himself quite unworthy of the promises which have been made to him.
Peter's unworthiness is shown by subtle ironies in the text. Peter, a moment ago the 'rock' on which the Church was to be built, is now a 'stumbling block', a rock not on which to build, not a firm foundation, but a rock that one might trip over, a hindrance.
And what of Peter's authority to bind and loose? Well, he's not slow in trying to use that authority. The first thing he says is, 'God forbid, Lord! This shall never happen to you.' Peter is here trying to bind on earth, to prevent the suffering and death of Christ, and so to bind in heaven.
But all this earns him is the Lord's stinging rebuke, 'Get behind me, Satan!' Peter has got it wrong, and any bubble of pride he had after receiving Jesus' praise a moment ago has been well and truly burst.
When Jeremiah said that 'the word of the Lord has become for me a reproach and derision', he was probably meaning that others, those hostile to the prophet, would mock and deride him. But in this week's gospel we see that when the prophet, or apostle, gets the message wrong, he will be insulted by the word of the Lord in the most literal way possible!
Of course, as we know, this was not the only occasion that the impetuous Peter gets things wrong. He messed up walking on the water; he gets Jesus wrong here; at the Transfiguration he offers to build tents for Jesus, Moses and Elijah; and, probably worst of all, when the Lord's passion is approaching he denies him, not once, but three times. Even much later, Peter has to be rebuked by St Paul for avoiding eating with Gentiles.
Peter, though, is no failure. It is true he had his failings, but he was no failure: under his leadership and preaching the Church spread, in Jerusalem, Judea and Samaria, and thence to Antioch, and to Rome, where he died a martyr's death.
If Peter is a man who achieved this much, and to whom were given great gifts and great authority, why do the Scriptures record all these failings? Why does the Bible make Peter a 'laughing stock' whom 'everyone mocks'?
It is, perhaps, to show us that Peter's great achievements were not his achievements alone; his gifts and authority were not just human gifts, but gifts of God. By the grace of God, such a man of failings is turned into a great success; the stumbling block is turned into the foundation stone.
Peter trusts in God's grace. He has to, for he cannot trust in himself. But the trust he has in God, the enthusiasm he has for the Gospel, helps him to pick himself up after his failures and today after his rebuke from Jesus. Peter, though he can get things wrong, does not give up, but perseveres in working for Christ.
We, like Peter, all have our failings. We fail to understand God's purpose as we ought, and we can fail in bearing witness to him. While we should be sorry for our failings and need to seek forgiveness for them, let us pray also for God's grace for ourselves, so that we may persevere in our faith, and in making that faith known to those we meet.
Indeed, let us proclaim the greatness of God in the great deeds he works through unworthy sinners such as ourselves.