At the time of his election to the See of Peter on 29 September 440, Leo expressed his gratitude for the trust placed in him by God and by God's Church ( Serm. 1). For some years thereafter he gave special sermons on the anniversary of his election, four of which have come down to us: Serms. 2 (441), 3 (443), 4 (444), and 5 (after 445?). The same general themes reverberate throughout the sermons: gratitude for the trust placed in him; humility resulting from his feelings of unworthiness and weakness, accompanied by a request for prayers that he prove competent; an appreciation for the magnitude of the work and the frequent occasions for stumbling presented by it; gratitude for the goodness, the unity, and the peace of his flock.
Fundamental to these sermons and to those on the feasts of Sts. Peter and Paul ( Serms. 82 and 83) is his praise of St. Peter, his belief in the primacy of Peter, and his reliance on Peter who continues to rule the Church through his own weakness. This same dependence on Peter is found at the conclusion of all his Lenten sermons, in his sermons on the fasts of September and December, and in his sermon on the Transfiguration ( Serm. 51). "Just as what Peter believed in Christ remains, so there remains what Christ instituted in Peter" ( Serm. 3.2).
This strong affirmation of the primacy held by the Bishop of Rome (made from the very beginning of his reign) underlines Leo's conviction that Christ intended Peter and his successors to be a unifying principle in the Church. In recognition of this, Pope John XXIII considered Leo to be "Doctor of the Church's Unity."1. "You are Peter and upon this rock I will build my Church, and the gates of hell will not prevail against it" (Mt 16.18). Serms. 4 and 83 contain his meditations on the words exchanged between Christ and St. Peter (Mt 16.16-19). It so happened that, during the fifth century when the fabric of Roman civilization was disintegrating, Peter's successor Leo was also the unifying principle of the world.____________________