Matthew applies to Jesus the prophetic oracle of Isaiah: “The
people who walked in darkness have seen a great light; upon those
who dwelt in the land of gloom a light has shone.” In like
manner, the psalmist’s assertion that the Lord is my light is
echoed in Jesus’ words: “I am the light of the world;
the man who follows me will have the light of life.”
Christianity is about light. Just as Jesus is light to the world, so his church is to be light in a world of darkness. Light enables the beauty of things to be seen, and we are called to make the beauty of the world shine. But we are not to call attention to ourselves: if you are aware of the light in a room, it is too bright. Our vocation is for others, not ourselves.
Surrounded by discrimination, oppression and war, Christians are challenged to “be united in mind and judgment,” and they are directed by God’s love to “bring humankind to unity and peace.” The light we bring to the world should lead in time to an end of all the world’s darkness: the alienation and oppression of peoples, the attacks on human life and dignity, and disrespect for God’s creation.
The Church ... serves as a leaven and as a kind of soul for human society as it is to be renewed in Christ and transformed into God’s family. ... the Church not only communicates divine life to men, but in some way casts the reflected light of that life over the entire earth.
Vatican II, Constitution on the Church in the Modern World,