47 The unity of the Church in time and space is linked to the unity of the faith: "there is one body and one Spirit… one faith" ( Eph 4:4-5). These days we can imagine a group of people being united in a common cause, in mutual affection, in sharing the same destiny and a single purpose. But we find it hard to conceive of a unity in one truth. We tend to think that a unity of this sort is incompatible with freedom of thought and personal autonomy. Yet the experience of love shows us that a common vision is possible, for through love we learn how to see reality through the eyes of others, not as something which impoverishes but instead enriches our vision. Genuine love, after the fashion of God’s love, ultimately requires truth, and the shared contemplation of the truth which is Jesus Christ enables love to become deep and enduring. This is also the great joy of faith: a unity of vision in one body and one spirit. Saint Leo the Great could say: "If faith is not one, then it is not faith". What is the secret of this unity? Faith is "one", in the first place, because of the oneness of the God who is known and confessed. All the articles of faith speak of God; they are ways to know him and his works. Consequently, their unity is far superior to any possible construct of human reason. They possess a unity which enriches us because it is given to us and makes us one. Faith is also one because it is directed to the one Lord, to the life of Jesus, to the concrete history which he shares with us. Saint Irenaeus of Lyons made this clear in his struggle against Gnosticism. The Gnostics held that there are two kinds of faith: a crude, imperfect faith suited to the masses, which remained at the level of Jesus’ flesh and the contemplation of his mysteries; and a deeper, perfect faith reserved to a small circle of initiates who were intellectually capable of rising above the flesh of Jesus towards the mysteries of the unknown divinity. In opposition to this claim, which even today exerts a certain attraction and has its followers, Saint Irenaeus insisted that there is but one faith, for it is grounded in the concrete event of the incarnation and can never transcend the flesh and history of Christ, inasmuch as God willed to reveal himself fully in that flesh. For this reason, he says, there is no difference in the faith of "those able to discourse of it at length" and "those who speak but little", between the greater and the less: the first cannot increase the faith, nor the second diminish it. Finally, faith is one because it is shared by the whole Church, which is one body and one Spirit. In the communion of the one subject which is the Church, we receive a common gaze. By professing the same faith, we stand firm on the same rock, we are transformed by the same Spirit of love, we radiate one light and we have a single insight into reality.
Source: Lumen Fidei (Vatican.va)