When Winston Churchill selected William Temple, a man with whom he disagreed politically, to be Archbishop of Canterbury he consoled himself by believing that he had chosen “the half crown in a penny bazaar”. It was in a not dissimilar spirit that many individuals and institutions interested in the Anglican Church cheered Rowan Williams's arrival in St Augustine's chair in 2003. Then the Archbishop of Wales, Dr Williams was known to have radical views but his supporters asserted, convincingly, that he was a figure of such intellectual and spiritual authority that he stood head and shoulders above any rival. In his short tenure as Archbishop of Canterbury, Temple did much to vindicate Churchill's first assessment. After five years, it has to be concluded that the same, alas, cannot be said for Dr Williams.

The needless storm that the Archbishop has started concerning Sharia is not, unfortunately, an isolated incident. It fits a pattern in which Dr Williams has entertained arguments in public that might have some virtue in the scholarly cut-and-thrust of the cloisters but that are at best naive and at worst destructive when placed in the public arena by the man who is the head not just of any church but the one established Church in this country. To muse, as he did, that: “An approach to law which simply said, ‘There is one law for everybody and that is all there is to be said'. I think that is a bit of a danger” is not only an extraordinary deduction but, actually, to borrow his words, a bit of a danger. If the purpose of this lecture was, as the Archbishop says, to assist community cohesion, then it not only failed in that effort but also backfired spectacularly.

The role of the modern Archbishop of Canterbury is, in fairness, a complicated one that has become considerably more demanding in the past two decades. The person concerned has to exercise direct control in the See of Canterbury itself, is responsible for the clear majority of bishoprics within the Church of England, is a significant figure within the formal machinery of the State, must be a public advocate for his faith and, crucially, has to lead the Anglican communion as a global institution with millions of followers.

Dr Williams has recognised these responsibilities. In his first press conference after his appointment, he felt a “calling to nourish a sense of proper confidence in the Church and more widely” and expressed the hope that during his time at the helm, “the years to come may see Christianity in this country able again to capture the imagination of our culture, to draw the strongest energies of our thinking and feeling into the exploration of what our creeds put before us”. Yet the blunt truth about his service as Archbishop is that the Church of England is suffering a deeper crisis of confidence than before and has less captured the imagination of our culture than, sometimes by his misjudgments, invited its contempt.

There have been many moments in the history of the Church when none of this would matter. Anglicanism could withstand an Archbishop of Canterbury who appeared to be eccentric in his opinions and ineffective in his ability to inspire and to unite his colleagues. This is not one of those moments. The General Synod will meet next week against a backdrop in which the Church is riven over its stance on homosexuality. This gathering will be a prelude to a Lambeth Conference later this year at which an understanding on this subject must be reached or fundamental consequences will occur. Anglicanism faces a de facto choice between an “open marriage” between its various factions, or a “trial separation”, especially between its African and North American brethren, or a complete divorce.

An accord that allowed Anglicanism to remain one church rather than fragment would be deeply desirable. If it is to be achieved, then an exceptional amount of tact and diplomacy will be required at the highest level to secure that prize. It is the task that should be the principal preoccupation of the Archbishop of Canterbury. This is precisely the wrong time for Dr Williams to be offering views at the very fringe of his role as a public figure and commentator, and which have an incendiary quality not only in terms of mainstream domestic sentiment but also particularly for the African Church, which has to manage a fraught relationship with Islam in awkward conditions. The Archbishop has, therefore, made his already Herculean challenge in maintaining the unity of Anglicanism harder by yet another foray on behalf of the wrong army on a mistaken battlefield.

This incident should prompt the Archbishop to think again about his approach in what may be, without exaggeration, the most fateful year for the Church of England and Anglicanism in its history. Aspiring to reach out to and better understand other faiths is an admirable ideal but it must take second place, for Dr Williams, to reaching out to and better understanding his own. He should not render himself mute but he has to choose his priorities with more acumen and has to weigh the probable impact of his statements considerably more forensically than is his habit.

There are already senior clergymen who have privately decided that it would be in the best interests of the Church if it were to be placed under new management. There are many more who believe that if a bargain can be struck at the Lambeth Conference, such a settlement would have more chance of survival if the Archbishop were to retire soon afterwards. If there is no sign that Dr Williams comprehends what his role should be, then it is, to borrow his odd contention about Sharia, “unavoidable” that his future will be questioned.

In many ways this is a personal as well as an institutional tragedy. This is an unusually intelligent Archbishop of Canterbury, who, while his lectures can be almost incomprehensible, is also capable of sermons that many find truly uplifting. He is a reformer in a Church where reform was and is necessary. He has wrestled with his conscience on the matter of homosexuality and determined that his duty as the head of the Anglican Church is to urge others to make concessions in this debate that he, if he were still an academic, would oppose. He is a decent man who retains many admirers. It would be foolish, nonetheless, for him not to recognise his failings and ask if he can overcome them. For if this Archbishop of Canterbury once was a half crown in a parade of old pennies, today he stands devalued.