You are the light of the world
"You are the light of the world. A city set on a hill cannot be hidden. Nor do men light a lamp only to put it under a bushel basket; they put it on a stand where it gives light to all in the house. The Lord called his disciples to salt of the earth because they seasoned with heavenly wisdom the hearts of men, rendered insipid by the devil. Now he calls them the fight of the world as well, because they have been enlightened by him, the true and everlasting light, and have themselves become a light in the darkness.
Since he is the Sun of Justice, he fittingly calls his disciples the light of the world. The reason for this is that through them, as through shining rays, he has poured out the light of the knowledge of himself upon the entire world. For by manifesting the light of truth, they have dispelled the darkness of error from the hearts of men.
Moreover, we too have been enlightened by them. We have been made light out of darkness as the Apostle says: For once you, were darkness, but now you are light in the Lord; walk as children of light. He says another time: For you are not sons of the light and of darkness, but you are all sons of light and of the day. Saint John also rightly asserts in his letter: God is light, and whoever abides in God is in the light just as God himself is in the light. Therefore, because we rejoice in having been freed from the darkness of error, we should always walk in the light as children of light. This is why the Apostle says: Among them you shine as lights in the world, holding fast to the word of life.
If we fail to live in the light, we shall, to our condemnation and that of others, be veiling over and obscuring by our infidelity the light men so desperately need. As we know from Scripture, the man who received the talent should have made it produce a heavenly profit, but instead he preferred to hide it away rather than put it to work and was punished as he deserved. Consequently, that brilliant lamp which was lit for the sake of our salvation should always shine in us. For we have the lamp of the heavenly commandment and spiritual grace, to which David referred: Your law, is a lamp to my feet and a light to my path. Solomon also says this about it: For the command of the law is a lamp.
Therefore, we must not hide this lamp of law and faith. Rather, we must set it up in the Church, as on a lamp stand, for the salvation of many, so that we may enjoy the light of truth itself and all believers may be enlightened."
From a treatise on the Gospel of Saint Matthew by Saint Chromatius, bishop (Tract. 5, 1. 3-4: CCL 9, 405-407)
Prepared by the Spiritual Theology Department