Catholicity means universality - a multiplicity that becomes unity; a unity that nevertheless remains multiplicity. From Paul's words on the Church's universality we have already seen that the ability of nations to get the better of themselves in order to look towards the one God, is part of this unity. In the second century, the founder of Catholic theology, St Irenaeus of Lyons, described very beautifully this bond between catholicity and unity and I quote him. He says: "The Church spread across the world diligently safeguards this doctrine and this faith, forming as it were one family: the same faith, with one mind and one heart, the same preaching, teaching and tradition as if she had but one mouth. Languages abound according to the region but the power of our tradition is one and the same. The Churches in Germany do not differ in faith or tradition, neither do those in Spain, Gaul, Egypt, Libya, the Orient, the centre of the earth; just as the sun, God's creature, is one alone and identical throughout the world, so the light of true preaching shines everywhere and illuminates all who desire to attain knowledge of the truth" (Adv. Haer. I 10, 2). The unity of men and women in their multiplicity has become possible because God, this one God of heaven and earth, has shown himself to us; because the essential truth about our lives, our "where from?" and "where to?" became visible when he revealed himself to us and enabled us to see his face, himself, in Jesus Christ. This truth about the essence of our being, living and dying, a truth that God made visible, unites us and makes us brothers and sisters. Catholicity and unity go hand in hand. And unity has a content: the faith that the Apostles passed on to us in Christ's name.
[Pope Benedict, 29 June 2005]