Why does God choose to put his Gospel into the physical form of the sacraments? Because it is in the very nature of faith to attach to what is not only heard but also seen. Pope Francis explains this dynamic in his recent encyclical Lumen Fidei:

30. The bond between seeing and hearing in faith-knowledge is most clearly evident in John’s Gospel. For the Fourth Gospel, to believe is both to hear and to see. Faith’s hearing emerges as a form of knowing proper to love: it is a personal hearing, one which recognizes the voice of the Good Shepherd (cf. Jn 10:3-5); it is a hearing which calls for discipleship, as was the case with the first disciples: “Hearing him say these things, they followed Jesus” (Jn 1:37). But faith is also tied to sight. Seeing the signs which Jesus worked leads at times to faith, as in the case of the Jews who, following the raising of Lazarus, “having seen what he did, believed in him” (Jn 11:45). At other times, faith itself leads to deeper vision: “If you believe, you will see the glory of God” (Jn 11:40). In the end, belief and sight intersect: “Whoever believes in me believes in him who sent me. And whoever sees me sees him who sent me” (Jn 12:44-45). Joined to hearing, seeing then becomes a form of following Christ, and faith appears as a process of gazing, in which our eyes grow accustomed to peering into the depths. Easter morning thus passes from John who, standing in the early morning darkness before the empty tomb, “saw and believed” (Jn 20:8), to Mary Magdalene who, after seeing Jesus (cf. Jn 20:14) and wanting to cling to him, is asked to contemplate him as he ascends to the Father, and finally to her full confession before the disciples: “I have seen the Lord!” (Jn 20:18).

How does one attain this synthesis between hearing and seeing? It becomes possible through the person of Christ himself, who can be seen and heard. He is the Word made flesh, whose glory we have seen (cf. Jn 1:14). The light of faith is the light of a countenance in which the Father is seen. In the Fourth Gospel, the truth which faith attains is the revelation of the Father in the Son, in his flesh and in his earthly deeds, a truth which can be defined as the “light-filled life” of Jesus.[24] This means that faith-knowledge does not direct our gaze to a purely inward truth. The truth which faith discloses to us is a truth centred on an encounter with Christ, on the contemplation of his life and on the awareness of his presence. Saint Thomas Aquinas speaks of the Apostles’ oculata fides — a faith which sees! — in the presence of the body of the Risen Lord.[25] With their own eyes they saw the risen Jesus and they believed; in a word, they were able to peer into the depths of what they were seeing and to confess their faith in the Son of God, seated at the right hand of the Father.

31. It was only in this way, by taking flesh, by sharing our humanity, that the knowledge proper to love could come to full fruition. For the light of love is born when our hearts are touched and we open ourselves to the interior presence of the beloved, who enables us to recognize his mystery. Thus we can understand why, together with hearing and seeing, Saint John can speak of faith as touch, as he says in his First Letter: “What we have heard, what we have seen with our eyes and touched with our hands, concerning the word of life” (1 Jn 1:1). By his taking flesh and coming among us, Jesus has touched us, and through the sacraments he continues to touch us even today; transforming our hearts, he unceasingly enables us to acknowledge and acclaim him as the Son of God. In faith, we can touch him and receive the power of his grace. Saint Augustine, commenting on the account of the woman suffering from haemorrhages who touched Jesus and was cured (cf. Lk 8:45-46), says: “To touch him with our hearts: that is what it means to believe”.[26] The crowd presses in on Jesus, but they do not reach him with the personal touch of faith, which apprehends the mystery that he is the Son who reveals the Father. Only when we are configured to Jesus do we receive the eyes needed to see him.