37 Those who have opened their hearts to God’s love, heard his voice and received his light, cannot keep this gift to themselves. Since faith is hearing and seeing, it is also handed on as word and light. Addressing the Corinthians, Saint Paul used these two very images. On the one hand he says: "But just as we have the same spirit of faith that is in accordance with scripture — ‘I believed, and so I spoke’ — we also believe, and so we speak" ( 2 Cor 4:13). The word, once accepted, becomes a response, a confession of faith, which spreads to others and invites them to believe. Paul also uses the image of light: "All of us, with unveiled faces, seeing the glory of the Lord as though reflected in a mirror, are being transformed into the same image" ( 2 Cor 3:18). It is a light reflected from one face to another, even as Moses himself bore a reflection of God’s glory after having spoken with him: "God… has shone in our hearts to give the light of the knowledge of the glory of God in the face of Christ" ( 2 Cor 4:6). The light of Christ shines, as in a mirror, upon the face of Christians; as it spreads, it comes down to us, so that we too can share in that vision and reflect that light to others, in the same way that, in the Easter liturgy, the light of the paschal candle lights countless other candles. Faith is passed on, we might say, by contact, from one person to another, just as one candle is lighted from another. Christians, in their poverty, plant a seed so rich that it becomes a great tree, capable of filling the world with its fruit.
Source: Lumen Fidei (Vatican.va)