The restoration of churchly unity among all Christians is the agreed aim of ecumenical work. It is the programme announced in the opening phrase—Unitas redintegratio— of the Vatican II Decree on Ecumenism (which is thus also the title). Such phraseology suggests that the unity to be restored did at some stage actually exist—that there was once, historically and empirically, an “undivided Church”, prior to the conflicts among Christians that gave rise to schisms and heresies. Reflective study of church history seems, however, to keep pushing further and further back to the point at which the Church was visibly and organically one. The Decree of Ecumenism dates the first of the divisions at the refusal of what was to become the Nestorian Church to accept the dogmatic pronouncement of the Council of Ephesus in the year 431. The convoking of that council had itself been an attempt to restore Christian unity. The further one goes back into history the more evident it becomes that it has always been necessary to restore unity. The “undivided Church” begins to look more like an endless task rather then any historically dateable empirical reality. The variety and complexity of divisions among Christians become increasingly obvious. It is not at all clear that anybody, either in the World Council of Churches or in the Catholic Church, not to mention the many other participants, has yet succeeded in working out a doctrine of the Church which does justice to the tangled anomalies of the situation.