Rarely has an archbishop been so tested. Only days before he attends the General Synod in York, Dr Rowan Williams has received a letter from more than 1,300 clergy, including 11 serving bishops, threatening to defect from the Church of England if women are consecrated bishops. The letter comes hard on the heels of an equally minatory ultimatum issued in Jerusalem last week by more than 250 bishops from across the Anglican Communion excoriating the Archbishop of Canterbury for his lack of moral leadership and calling on traditionalists to “sideline” him.

That is not all. In a challenge to his authority as primus inter pares, some 800 Church of England clergy and lay leaders took the first step on Tuesday to forming a “Church within a Church”. Led by three overseas bishops, the group met in a London evangelical church to assert their opposition to the ordination of gay people as well as an anathema on liberal theology that they said was undermining the Church.

Most are expected to become part of the Fellowship of Confessing Anglicans (Foca), formed at a meeting in Jerusalem by the Global Anglican Future Conference (Gafcon). It bears out Dr Williams's fear that a schism in the Anglican Communion would lead not just to the exodus of several churches from the 38-strong body but to irreparable fissures within national churches.

Since one of the primary roles of his office is to hold together the communion, Dr Williams has therefore responded in unusually forthright language to Gafcon and to Foca. He warned the rebels to “think very carefully about the risks entailed” and told them that a self-selected group from among the primates would not pass the test of legitimacy for all in the 80-million-strong communion. Their claim to operate across church boundaries was “fraught with difficulty”, both because of the commitment of each church to recognise the other and because of the practical difficulties of huge geographical and cultural divides.

Dr Williams went farther. Recognising, at last, that this row challenges the very legitimacy of his office, he asked the rebels by what authority the Gafcon primates are to be deemed acceptable and how “effective discipline” is to be maintained in a situation of competing jurisdictions. He also suggested that some clergy, especially in developing countries, were hiding behind theological arguments to conceal personal problems and disagreements that had nothing to do with the row over homosexuality and liberal practice.

His riposte is as timely as it is pertinent. There is a narrowness, self-righteousness and arrogance about some of the rebels that is deeply unappealing. Several want to have it both ways: to remain within the communion (largely because of the legal and property obstacles that arise from a walkout) while sniping at Canterbury's authority. The more immediate challenge this weekend, however, comes not from Foca but from clergy unreconciled to women bishops. They want permanent, binding safeguards for traditionalists that Dr Williams and others are unwilling to concede for fear of enshrining discrimination. He must therefore address their defiance in York as vigorously as he has replied to the Gafcon rebels. On his performance hangs not only the unity of the Church of England but the prospects for the fractious Lambeth Conference. Rarely has a challenge been as daunting.