On Thursday, Dr Rowan Williams, Archbishop of Canterbury, makes his own journey to Rome. It's just a long-standing two-day business trip, you understand. He's not taking the Church of England with him. But I've been speaking to Anglican bishops who ask, wide-eyed, why he's going at all, so soon after the Pope's dawn raid on English Anglo-Catholics.

One of our senior bishops wanted to know why Dr Williams was bounced into attending the press conference, called hastily to announce Benedict's takeover bid - and staged alongside his top man in England, Archbishop Vincent Nichols - in the Roman Church's London gaff. Why hadn't Dr Williams, this bishop wanted to know, just instructed his chief spin doctor to say airily that he was busy and the Pope's acquisitive ambitions weren't a matter for him? Why didn't he also let it be known that he would no longer be going to Rome this week, because he and the Pontiff no longer had ecumenical common ground to discuss?

None of that would be in Dr Williams's nature, of course. We'd be entering an alternative reality in which Harrison Ford plays him in the movie, bursting into Benedict's Vatican chamber and hissing: "What's your game, Benny?" The Pope (played by Rutger Hauer) slowly turns: "Vot do you sink I'm doing, unshaven one? For you, ze Communion is over."

Back in the real world later this week, Dr Williams will address an ecumenical conference, poignantly enough, and will have a private meeting with Benedict, who always gives the Archbishop plenty of his time in Rome, if not notice of his intentions for English Catholics.

Tom Butler, Bishop of Southwark, said yesterday that he hoped Dr Williams would tell the Pope he was "disappointed". That's perfectly Anglican: understated and anodyne.

But if Dr Williams has been determined not to make a drama out of this crisis, the Pope has been playing to the gallery. And it's his strategy we should be focused on. Unlike the Bishop of Southwark, I don't want Dr Williams to express his "disappointment", I want him to ask, perhaps a little more politely than in my movie storyboard, what the Pope's game is.

I'll tell you what I think it is. Benedict is determined to rebuild his one, true and, importantly, universal Church. Under a banner of doctrinal purity, he is annexing orthodoxy. That's why, in 2006, he dropped the title Patriarch of the West from the list of handles conferred on the Pope. Too parochial, too implicitly restrictive. Supreme Pontiff of the Universal Church is altogether more agreeable to him.

Organic growth is not an option for him, so he must grow the Church by acquisition. The Apostolic Constitution is part of that acquisitive strategy, aimed less at dismembering the Church of England than at bringing home Catholics in America and Australia; but even more significantly being tested as a tool ahead of bringing others, such as the ultra-traditionalist and schismatic Lefebvrists, back into the fold. Benedict wants to consolidate orthodoxy wherever he finds it and, eastern Orthodox patriarchs should note, there can be only one voice of authority and it speaks from Rome. This Pope is on a reactionary and Counter-Reformational rampage.

Dr Williams might like to reflect on that, if he has a moment to stare into a Bellini in Rome. His own Anglican Communion is doing the opposite of consolidating its orthodoxy. For most of us, that's fine: I have "Anglican" written though me like a stick of rock, though some might call it a bag of fudge. By contrast, Dr Williams has "Anglicanism" written on his heart and will go to his grave, I imagine, struggling to hold the worldwide communion together.

But the game must now be for the Church of England to square up to a newly rampant Rome. As it happens, I don't think many Anglo-Catholics will go across, but that's not the point. What we're facing is a vigorous challenge to our established Church's doctrinal authority in our own country. I'll say this to the former Patriarch of the West, even if Dr Williams won't: Come and have a go if you think you're hard enough. We have, after all, been here before.

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