IN February of 2008, the archbishop of Canterbury, the Most Rev. Rowan Williams, delivered an address on the fraught subject of “Islam in English Law.” The speech, which circled around the question of whether a civil justice system could accommodate Islamic legal codes, was learned, recondite and occasionally impenetrable. The headlines it generated were not: The head of the Church of England, the newspapers blared, had endorsed Shariah law in Britain. Which he had — sort of, kind of, in the most academic and nuanced and trying-not-to-offend-anyone way imaginable.

For Williams, who announced on Friday that he will lay down his mitre at the end of this year, the Shariah controversy was typical of his tenure. Bearded, kindly and theologically subtle, the...