Praised be Jesus and Mary, now and forever.
Our readings need some context since we just got dropped back into the cycle of readings for Ordinary Time after Pentecost. The Gospel today is drawn from the Sermon on the Mount, which we would have started yesterday if it hadn't been a feast. And the first reading would probably need context, even if we had heard the previous reading on Monday. It comes from a section of the letter in which St. Paul explains why he had not come to visit the Corinthians as he had planned. It was not because he was vacillating, not because he was saying yes and no at the same time to visit in Corinth, but rather, as God is faithful, our word to you is not yes and no.
This is where the passage we heard begins. St. Paul is an apostle of Christ Jesus, and as God means what He says, so too does St. Paul, who speaks on God's behalf. St. Paul is faithfully salt of the earth and light of the world. He is communicating what God communicates, and this applies to everything that St. Paul says, including the matter about his plans that he had to change, but it applies especially to the gospel. Jesus is God's yes and amen to all of His promises, be the fulfillment of revelation.
And God has not only given us His Word, Jesus has not only spoken His Word, but the Father and the Son have sent us the Holy Spirit, poured out first at Pentecost, and then when the Christians of Corinth were baptized and confirmed, and more recently upon each of us in our own baptism and confirmation. The Holy Spirit is, St. Paul says, the first installment of the intimacy with God that we will have in heaven. And so we do not need to wait for heaven. We wait for the fullness that we will have in heaven, but already here on earth we can enjoy intimacy with God. Already here we have God dwelling in our soul.
In the gospel, we heard Christ's own words. He's speaking about the gospel, and He expresses it, speaking especially about spreading the gospel, spreading the news of the kingdom of God. And He expresses this with two parables or figures, salt and light. These are directed primarily at the apostles, bishops, priests, and deacons who communicate the wisdom of the gospel, especially by preaching it. But they also apply to ordinary Christians insofar as they too, every Christian, must communicate the faith in some way. Not everyone is good at explaining it, but everyone can and should communicate it by the example of his life.
So Christ says first we must be the salt of the earth. Salt gives flavor and it preserves from spoiling. And this is what we must do by communicating the wisdom of the gospel to those who do not know it or do not appreciate it. The Responsorial Psalm spoke of appreciation for God's Word, spoke of its value. "The revelation of your words sheds light. It gives understanding to the simple." And it is really amazing. "I gasp with open mouth in my yearning for your commands."
So salt gives flavor to food. Without salt, food does not have a pleasing taste. And unless you put the gospel into practice, you will not be pleasing to God. You can't see where there is salt in food. And so, similarly, the life of those who live the gospel looks much like the life of everyone else. They go to work, they eat, they sleep, they have children, and they raise them. But the taste of salted food is different. And God sees those same actions differently when they're motivated by faith and charity instead of self-interest. Faithful Catholics pray and take care to do everything according to God's law.
Salt also keeps food from spoiling. It was used especially for this in the past to preserve food for the long term. And so living the faith keeps us from spoiling. It keeps us from going bad. It prevents every kind of moral corruption. There are certainly unfaithful and corrupt Christians, even though they have received that salt of the Word by preaching. But this shows that they lack it in the sense that they're not committed to living the faith. The problem is not that they do not know the gospel message, but that they do not put it into practice.
Christ also gives a warning here. If salt does not keep its savor, if it does not make things salty, it is useless and will be trampled underfoot like stone or sand. Likewise, someone who fails to teach the faith through word or example as he must deserves to be trampled underfoot. These are hard words, but Christ spoke them with love to admonish those who have the task of teaching the faith.
Christians are also the light of the world. And this may be surprising because Christ said that He is the light of the world. But all of us become the light of the world when we spread to others the light that we have received from Christ. For the world is not enlightened so much directly by Christ today. He is not walking around and preaching, but rather it is through the Church, through Christians, that Christ's teaching is communicated. And the task of teaching belongs especially to the clergy, but also all the faithful have the duty to spread the faith according to their abilities. Parents can teach the faith to their children, and grandparents can teach the faith to their grandchildren, especially when the parents fail to do so. Even when someone is not good at explaining things, they can still teach by good example and with good deeds, as our translation puts it.
And so Christ wants us to be the light of the world, especially by the way we live. So the intention is important. Christ says here, "Your light must shine before others so that they may see your good deeds." Even though He says elsewhere that we should not do our good deeds to be seen. So no, we should not do them in order to be seen and praised. But we should simply live as Christians, live as true followers of the gospel, and thereby let others see and learn from us what it is to live the gospel. Let us preach then by our life.
And in the parable of salt, so also here Christ admonishes us to be faithful to what He has made us, to what we have become by His grace. One does not light a lamp and then put it under a bushel basket. So if Christ has made us the light of the world, it is so that we can enlighten the world. If we hide our life and so if we act as though we're not Christian, if we fail to speak of our faith, then what good did it do for Christ to make us light? May the world not continue to remain in darkness and blindness because we lack the courage to give it the light it needs. 
Praised be Jesus and Mary, now and forever.