Firmly rooted in the biblical doctrine of creation, Irenaeus refuted the Gnostic dualism and pessimism which debased corporeal realities. He decisively claimed the original holiness of matter, of the body, of the flesh no less than of the spirit. But his work went far beyond the confutation of heresy: in fact, one can say that he emerges as the first great Church theologian who created systematic theology; he himself speaks of the system of theology, that is, of the internal coherence of all faith. At the heart of his doctrine is the question of the "rule of faith" and its transmission. For Irenaeus, the "rule of faith" coincided in practice with the Apostles' Creed, which gives us the key for interpreting the Gospel, for interpreting the Creed in light of the Gospel. The Creed, which is a sort of Gospel synthesis, helps us understand what it means and how we should read the Gospel itself.
The following is a quote from an exquisite text by this saint about Christ's incarnation and overflowing life manifested in us, especially this famous line: "life in man is the glory of God". It is part of the Office of Readings for today.
The Word became the steward of the Father’s grace for the advantage of men, for whose benefit he made such wonderful arrangements. He revealed God to men and presented men to God. He safeguarded the invisibility of the Father to prevent man from treating God with contempt and to set before him a constant goal toward which to make progress. On the other hand, he revealed God to men and made him visible in many ways to prevent man from being totally separated from God and so cease to be. Life in man is the glory of God; the life of man is the vision of God. If the revelation of God through creation gives life to all who live upon the earth, much more does the manifestation of the Father through the Word give life to those who see God.