A phrase of S Paul, in one of the earliest documents of the Church's Magisterium, was, we have seen taken up by S Vincent of Lerins in his insistence that development in Doctrine must be eodem sensu eademque sententia. In the last couple of centuries it has been transformed, by repetition, into a central plank of the Magisterium. Two Ecumenical Councils and a succession of Roman Pontiffs have done this. You will find it in Ineffabilis Deus, by which in 1854 S Pius IX defined the dogma of the Immaculate Conception. It appears in the Dogmatic Constitution of Vatican I Dei filius (at the end, just before the anathemas). S Pius X's Pascendi Dominici gregis repeats (para 28) these words of Dei filius in its treatment of Modernism, and the phrase was incorporated into the Anti-Modernist Oath taken by all clergy until 1967. After S John XXIII used it in his highly significant and programmatic Address at the start of Vatican II, it was repeated in Gaudium et spes (para 62), and S John Paul II, interestingly, extended its use from Dogmatic to Moral Theology in Veritatis splendor (para 53). And, if the Rule of Believing really is established by the Rule of Praying, then eodem sensu eademque sententia is right at the heart, not only of Vatican II, but also of the 'Spirit of Vatican II' as enunciated by the post-Conciliar liturgical changes: the crucial passage from the Commonitorium of S Vincent of Lerins is ordered to be read each year in the Liturgia Horarum (Week 27 of the Year, Friday). It is not surprising that Pope Benedict cited these words in his programmatic Address to the Roman Curia in 2005.
Fifteen hundred years ago ... and, if the world endures, fifteen hundred year from now, when Pope Francis XVI during some crisis or other is busily writing a Post-Synodal Exhortation ... it was and will be as true as it is today that the Deposit of Faith, the Tradition handed on through the Apostles, can only ever exist, can only ever be expressed, so that it comes to Christ's People with the same sense and with the same meaning.