From: Matthew 5:13-16
Salt of the Earth and Light of the World
(Jesus said to the multitude:)  "You are the salt of the earth; but if salt has lost its taste, how shall its saltness be restored? It is no longer good for anything except to be thrown out and trodden under foot by men.
 "You are the light of the world. A city set on a hill cannot be hid.  Nor do men light a lamp and put it under a bushel, but on a stand, and it gives light to all in the house.  Let your light so shine before men, that they may see your good works and give glory to your Father who is in Heaven."
13-16. These verses are a calling to that apostolate which is part and parcel of being a Christian. Every Christian has to strive for personal sanctification, but he also has to seek the sanctification of others. Jesus teaches us this, using the very expressive simile of salt and light. Salt preserves food from corruption; it also brings out its flavor and makes it more pleasant; and it disappears into the food; the Christian should do the same among the people around him.
"You are salt, apostolic soul. `Bonum est sal': salt is a useful thing', we read in the holy Gospel; `si autem sal evanuerit': but if the salt loses its taste', it is good for nothing, neither for the land nor for the manure heap; it is thrown out as useless. You are salt, apostolic soul. But if you lose your taste..." ([St] J. Escriva, "The Way", 921).
Good works are the fruit of charity, which consists in loving others as God loves us (cf. John 15:12). "I see now", St. Therese of Lisieux writes, "that true charity consists in bearing with the faults of those about us, never being surprised at their weaknesses, but edified at the least sign of virtue. I see above all that charity must not remain hidden in the bottom of our hearts: `nor do men light a lamp and put it under a bushel, but on a stand, and it gives light to all in the house.' It seems to me that this lamp is the symbol of charity; it must shine out not only to cheer those we love best but all in the house" ("The Autobiography of a Saint", Chapter 9).
Apostolate is one of the clearest expressions of charity. The Second Vatican Council emphasized the Christian's duty to be apostolic. Baptism and Confirmation confer this duty, which is also a right (cf. "Lumen Gentium", 33), so much so that, because the Christian is part of the mystical body, "a member who does not work at the growth of the body to the extent of his possibilities must be considered useless both to the Church and to himself" ("Apostolicam Actuositatem", 2). "Laymen have countless opportunities for exercising the apostolate of evangelization and sanctification. The very witness of a Christian life, and good works done in a supernatural spirit, are effective in drawing men to the faith and to God; and that is what the Lord has said: "Let your light so shine before men, that they may see your good works and give glory to your Father who is in Heaven" ("Apostolicam Actuositatem", 6).
"The Church must be present to these groups [those who do not even believe in God] through those of its members who live among them or have been sent to them. All Christians by the example of their lives and witness of their word, wherever they live, have an obligation to manifest the new man which they put on in Baptism, and to reveal the power of the Holy Spirit by whom they were strengthened at Confirmation, so that others, seeing their good works, might glorify the Father and more perfectly perceive the true meaning of human life and the universal solidarity of mankind" ("Ad Gentes", 11; cf. 36).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.