The opening words of the official English translation are: "The restoration of unity among all Christians is one of the principal concerns of the Second Vatican Council."
Unitatis acknowledges that there are serious problems facing prospects of reunion with Reformation communities that make no attempt to claim apostolic succession as the Anglican communion does.
Ecclesial communities which adhere to Calvinism are a particularly challenging case because they and Catholicism have important doctrinal differences on key issues such as ecclesiology, liturgy and mariology.
Pope John Paul II refers to and builds on the teaching of Unitatis Redintegratio in his encyclical letter of 25 May 1995, Ut unum sint.
Cardinal Walter Kasper discussed the status of the problems by the document on the 40th anniversary of the promulgation of Unitatis in remarks entitled "The Decree on Ecumenism – Read Anew After Forty Years".