In celebrating Peter and Paul by a common solemnity, we're celebrating the apostolicity of the Church - the rootedness of the Church in a commission to the apostles.
More especially, we're celebrating the apostolicity of the Roman church, the church of the city of Rome, which these two apostles confirmed in the faith by the testimony of their preaching and martyrdoms. In the West, this is the only apostolically founded see there is, and so it's the channel of apostolicity to all the others.
In and of themselves, there'd be no particular reason to bring Peter and Paul together. Peter, virtually the first apostle to be chosen, was destined to be the guardian of the revelation Christ had brought - and as protector of the apostolic deposit, then, the human foundation of the Church's faith. Being converted, he will go on to 'confirm' (in faith) his brethren.
Paul is coming from somewhere else. Paul is the last apostle to be called and, by his own admission, he was 'born' out of time - that is, after the public series of resurrection appearances had ended. His role is quite different from Peter's. He was the apostle of the pagans, someone divinely authorised to modify the received understanding of the Gospel so as to ease the entry of non-Jews into the Church. He is a radical or 'prophetic' figure, then, whose adventurous policies could only be justified, if at all, by their missionary fruits.
Peter the guardian, Paul the prophet: that's how it seems to be. These are simplifications, admittedly. We know from Paul's letters, for example, of his concern for the integrity of the Gospel tradition. The contrast may be a simplification, then, but it's not an altogether misleading one. The tension between Peter and Paul in the Acts of the Apostles shows that.
What brings together liturgically Peter and Paul, and not, for instance, Peter and Andrew or Paul and Barnabas, is their role in the founding of the primatial see of the West. That's the see that is also, so Catholic Christians say, the universally primatial see of the Christian world as a whole.
Although we think of the Roman church and bishop mainly in Petrine terms - naturally enough, when matters like power and teaching authority in the community are at stake, for these are specifically Petrine concerns - still, the Roman church speaks of itself as both Petrine and Pauline. It regards itself as created by the preaching and witness of these two apostles together.
The Petrine factor in the apostolicity of the Roman church has to do with conservation. It's a matter of holding on to what Christ taught, and how the apostles understood what he did and what he was - all on the presupposition, of course, that Christ is God-incarnate and that his coming therefore represents the moment of supreme illumination of human experience.
Peter gathers the Church in identity of faith around himself, just as people of all nationalities go on pilgrimage to the city of Rome which in that way is an image of the New Jerusalem of the end of time.
The Pauline factor in Rome's apostolicity, by contrast, is about mission - about outward expansion and necessary adaptation. It's a matter of maintaining the momentum of the impulse given by the Spirit at Pentecost, which caused the apostles to speak in a variety of tongues and drove out the Church onto the highways and byways of the contemporary world.
And that is all on the presupposition that the Spirit really is Christ's Spirit, the Spirit of the Son, and so is the release of the energy and liberty of God himself into the world. Paul sends out the Church on mission, adapting its message to the styles of different cultures just as Rome has been, in Christian history, an ever-renewed source of missionary impulse, symbolised today in the world-wide journeyings of Pope John Paul II.
The Church has to be bothguardian and prophet. A pope is called to fulfil that in a sometimes spectacular way. The grace offered us on this feast is that a bit of that combination can rub off on us as well.