From: Luke 3:1-6
The Preaching of John the Baptist
 In the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of the region of Ituraea and Trachonitis and Lysanias tetrarch of Abilene,  in the high-priesthood of Annas and Caiaphas, the word of God came to John the son of Zechariah in the wilderness;  and he went into all the region about the Jordan, preaching a baptism of repentance for the forgiveness of sins.  As it is written in the book of the words of Isaiah the prophet, "The voice of one crying in the wilderness: Prepare the way of the Lord, make his paths straight.  Every valley shall be filled, and every mountain and hill shall be brought low, and the crooked shall be made straight, and the rough ways shall be made smooth;  and all flesh shall see the salvation of God."
1. The Gospel identifies very precisely the time and place of the public appearance of John the Baptist, the Precursor of Christ, "Tiberius Caesar" was the second emperor of Rome, and the fifteenth year of his reign corresponds to A.D. 27 or 29, depending on which of the two possible calculations is correct.
"Pontius Pilate" was governor or "praefectus" of Judea from A.D. 26 to 36. His jurisdiction also extended to Samaria and Idumea.
The "Herod" referred to here is Herod Antipas, a son of Herod the Great, who succeeded to part of his father's territory with the title of tetrarch, not king. "Tetrarch" indicated that he exercised his power in subordination to Roman authority. It was Herod Antipas, who died in A.D. 39, who had St John the Baptist beheaded. On the identity of the four Herods in the New Testament, see the note on Mt 2:1.
"Philip", another son of Herod the Great and stepbrother of Herod Antipas, was tetrarch in the territory mentioned here up to the year 34 B.C. He married Herodias, who is spoken about in Mk 6:17-19.
2. The high priest at the time was "Caiaphas", who held the position from A.D. 18 to 36. Annas, his father-in-law, was still so influential that he was considered as the "de facto" head of Jewish religious and political life. That is why, when Christ was arrested, he was first interrogated before Annas (Jn 18:12-24). St Luke therefore is perfectly justified in calling him the high priest.
2-3. Here St Luke formally introduces St John the Baptist, who appears in his gospel a number of times. When Christ praises the Baptist (cf. Mt 11: 7-9) he refers particularly to his strength of will and his commitment to his God-given mission. Humility, austerity, courage and a spirit of prayer figure strongly in John's personality. So faithful was he to his mission of preparing the way for the Messiah that Christ praises him in a unique way: he is the greatest of those born of woman (cf. Mt 11:11), "a burning and shining lamp" (Jn 5:35). He burned with love, and shone by the witness he bore. Christ "was the light" (Jn 1:9); the Baptist "came for testimony, to bear witness to the light, that all might believe through him" (Jn 1:7).
John the Baptist appears on the scene preaching the need for repentance. He prepares "the way of the Lord". He is the herald of salvation: but his mission does not go beyond that; he simply announces that salvation is coming. "Among you stands one...who comes after me, the thong of whose sandal I am not worth to untie" (Jn 1:27). He points Christ out: "Behold, the Lamb of God" (Jn 1:29, 36), behold "the Son of God" (Jn 1:34); and he rejoices to see his own disciples leave him to follow Christ (Jn 1:37): "He must increase, but I must decrease" (Jn 3:30).
4-6. In the second part of the Book of Isaiah (chaps. 40-55), which is called the "Book of the Consolation of Israel", the Jewish people are told that they will once again suffer exile and a new exodus in which their guide will be, not Moses, but God himself; once again they will make their way through the desert to reach a new promised land. St Luke sees the preaching of the Baptist, who announces the arrival of the Messiah, as fulfilling this prophecy.
Because the Lord is imminent, people must prepare themselves spiritually, by doing penance for their sins, to receive the special divine grace the Messiah is bringing. This is what he means by levelling the mountains and making the Lord's path straight.
Every year in its Advent liturgy the Church proclaims the coming of Jesus Christ, our Savior, exhorting every Christian to purify his or her soul by a new interior conversion.
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.