18 This fullness which Jesus brings to faith has another decisive aspect. In faith, Christ is not simply the one in whom we believe, the supreme manifestation of God’s love; he is also the one with whom we are united precisely in order to believe. Faith does not merely gaze at Jesus, but sees things as Jesus himself sees them, with his own eyes: it is a participation in his way of seeing. In many areas in our lives we trust others who know more than we do. We trust the architect who builds our home, the pharmacist who gives us medicine for healing, the lawyer who defends us in court. We also need someone trustworthy and knowledgeable where God is concerned. Jesus, the Son of God, is the one who makes God known to us (cf. Jn 1:18). Christ’s life, his way of knowing the Father and living in complete and constant relationship with him, opens up new and inviting vistas for human experience. Saint John brings out the importance of a personal relationship with Jesus for our faith by using various forms of the verb "to believe". In addition to "believing that" what Jesus tells us is true, John also speaks of "believing" Jesus and "believing in" Jesus. We "believe" Jesus when we accept his word, his testimony, because he is truthful. We "believe in" Jesus when we personally welcome him into our lives and journey towards him, clinging to him in love and following in his footsteps along the way. To enable us to know, accept and follow him, the Son of God took on our flesh. In this way he also saw the Father humanly, within the setting of a journey unfolding in time. Christian faith is faith in the incarnation of the Word and his bodily resurrection; it is faith in a God who is so close to us that he entered our human history. Far from divorcing us from reality, our faith in the Son of God made man in Jesus of Nazareth enables us to grasp reality’s deepest meaning and to see how much God loves this world and is constantly guiding it towards himself. This leads us, as Christians, to live our lives in this world with ever greater commitment and intensity.
Source: Lumen Fidei (Vatican.va)