The Church of England has survived the Spanish Armada, the English Civil War and Elton John performing “Candle in the Wind” at Princess Diana’s Westminster Abbey funeral. So it will probably survive the note the Vatican issued last week, inviting disaffected Anglicans to head Romeward, and offering them an Anglo-Catholic mansion within the walls of the Roman Catholic faith.

But the invitation is a bombshell nonetheless. Pope Benedict XVI’s outreach to Anglicans may produce only a few conversions; it may produce a few million. Either way, it represents an unusual effort at targeted proselytism, remarkable both for its concessions to potential converts — married priests, a self-contained institutional structure, an Anglican...