From: Luke 1:67-79
 And his father Zechariah was filled with the Holy Spirit, and prophesied, saying,  "Blessed be the Lord God of Israel, for he has visited and redeemed his people,  and has raised up a horn of salvation for us in the house of his servant David,  as he spoke by the mouth of his holy prophets from of old,  that we should be saved from our enemies, and from the hand of all who hate us;  to perform the mercy promised to our fathers, and to remember his holy covenant,  the oath which he swore to our father Abraham,  to grant us that we, being delivered from the hand of our enemies, might serve him without fear,  in holiness and righteousness before him all the days of our life.  And you, child, will be called the prophet of the Most High; for you will go before the Lord to prepare his ways,  to give knowledge of salvation to his people in the forgiveness of their sins,  through the tender mercy of our God, when the day shall dawn upon us from on high  to give light to those who sit in darkness and in the shadow of death, to guide our feet into the way of peace."
67. Zechariah, who was a righteous man (cf. v. 6), received the special grace of prophecy when his son was born--a gift which led him to pronounce his canticle, called the "Benedictus", a prayer so full of faith, reverence and piety that the Church has laid it down to be said daily in the Liturgy of the Hours. Prophecy has not only to do with foretelling future events; it also means being moved by the Holy Spirit to praise God. Both aspects of prophecy are to be found in the "Benedictus".
68- 79. Two parts can be discerned in the "Benedictus": in the first (vv. 68-75) Zechariah thanks God for sending the Messiah, the Savior, as he promised the patriarchs and prophets of Israel.
In the second (vv. 76-79) he prophesies that his son will have the mission of being herald of the Most High and precursor of the Messiah, proclaiming God's mercy which reveals itself in the coming of Christ.
72-75. Again and again God promised the patriarchs of the Old Testament that he would take special care of Israel, giving them a land which they would enjoy undisturbed and many descendants in whom all the peoples of the earth would be blessed. This promise he ratified by means of a covenant or alliance, of the kind commonly made between kings and their vassals in the Near East. God, as Lord, would protect the patriarchs and their descendants, and these would prove their attachment to him by offering him certain sacrifices and by doing him service. See, for example, Genesis 12:13; 17:1-8; 22:16-18 (God's promise, covenant and pledge to Abraham); and Genesis 5:11-12 (where he repeats these promises to Jacob). Zechariah realizes that the events resulting from the birth of John his son, the Precursor of the Messiah; constitute complete fulfillment of these divine purposes.
78-79. The "dawning", the "dayspring", is the Messiah, Jesus Christ, coming down from heaven to shed his light upon us: "the son of righteousness shall rise, with healing on its wings" (Mal 4:2). Already in the Old Testament we were told about the glory of the Lord, the reflection of his presence--something intimately connected with light. For example, when Moses returned to the encampment after talking with God, his face so shone that the Israelites "were afraid to come near him" (Ex 34:30). St John is making the same reference when he says that "God is light and in him there is no darkness" (1 Jn 1:5) and that there will be no light in heaven "for the glory of God is its light" (cf. Rev 21:23; 22:5).
The angels (cf. Rev 1:11) and the saints (cf. Wis 3:7; Dan 2:3) partake of this divine splendor; our Lady does so in a special way. As a symbol of the Church she is revealed to us in the Apocalypse as "clothed with the sun, with the moon under her feel, and on her head a crown of twelve stars" (12:1).
Even when we live in this world, this divine light reaches us through Jesus Christ who, because he is God, is "the true light that enlightens every man" (Jn 1:9), as Christ himself tells us: "I am the light of the world; he who follows me will not walk in darkness" (Jn 8:12).
Such is Christians' share in this light of God that Jesus tells us: "You are the light of the world" (Mt 5:14). Therefore, we must live as children of the light (cf. Lk 16:8), whose fruit takes the form of "all that is good and right and true" (Eph 5:9); our lives should shine oul, thereby helping people to know God and give him glory (cf. Mt 5:16).
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.