From: Matthew 6:19-23
Trust in God's Fatherly Providence
(Jesus said to His disciples,)  "Do not lay up for yourselves treasures on earth, where moth and rust consume and where thieves break in and steal,  but lay up for yourselves treasures in Heaven, where neither moth nor rust consumes and where thieves do not break in and steal.  For where your treasure is, there will your heart be also.
 "The eye is the lamp of the body. So, if your eye is sound, your whole body will be full of light;  but if your eye is not sound, your whole body will be full of darkness."
19-21. The idea here is very clear: man's heart yearns for a treasure which will give him security and happiness. However, every treasure in the form of earthly goods--wealth, property--becomes a constant source of worry, because there is always the risk we will lose it or because the effort to protect it is such a strain.
Against this, Jesus teaches us here that our true treasure lies in good works and an upright life, which will be eternally rewarded by God in Heaven. That indeed is treasure which one never loses, a treasure on which Christ's disciple should put his heart.
Jesus closes the teaching contained in the preceding verses with a kind of refrain (verse 21). He is not saying that people should be unconcerned about earthly things; what He does say is that no created thing can be "the treasure", the ultimate aim, of man. What man should do is make his way to God, sanctify himself and give all glory to God, by making right use of the noble things of the earth: "Whether you eat or drink, or whatever you do, do all to the glory of God" (1 Corinthians 10:31; cf. Colossians 3:17).
22-23. Here is another jewel of Jesus' wisdom teaching. It begins with a sentence which is then immediately explained. The Master uses the simile of the eye as a lamp which provides the body with light. Christian exegesis has seen this "eye", this "lamp", as meaning the motivation behind our behavior. St. Thomas explains it in this way: The eye refers to motive. When a person wants to do something, he first forms an intention: thus, if your intention is sound--simple and clear--that is to say, if it is directed towards God, your whole body, that is, all your actions, will be sound, sincerely directed towards good" (St. Thomas Aquinas, "Commentary on St. Matthew", 6, 22-23).
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.