It is taken for granted today among theologians that the principal achievement of the Second Vatican Council was its Dogmatic Constitution on the Church, Lumen Gentium. It is beginning to be equally taken for granted that the core of this central document is its vision of Church as communion. "Communion, " Pope John Paul II has said, "is the very mystery of the Church." Church: A Spirited Communion grows out of the ecclesiology of the Council as a systematic treatment of this notion of communion. Church: A Spirited Communion is not, however, a book only about the Church. It is a book about the God whom Christians confess as Triune, who calls the Church into existence and who seeks its commitment in every age. It is a book about the Church only to the extent that the Church is in communion with this God. To the extent that it is a book which is primarily theo-logical and only secondarily ecclesio-logical, it is a book which adheres to God rather than to men and summons the Church to do the same.