522 The coming of God’s Son to earth is an event of such immensity that God willed to prepare for it over centuries. He makes everything converge on Christ: all the rituals and sacrifices, figures and symbols of the “First Covenant”.195 He announces him through the mouths of the prophets who succeeded one another in Israel. Moreover, he awakens in the hearts of the pagans a dim expectation of this coming.
523 St. John the Baptist is the Lord’s immediate precursor or forerunner, sent to prepare his way.196 “Prophet of the Most High”, John surpasses all the prophets, of whom he is the last. 197 He inaugurates the Gospel, already from his mother’s womb welcomes the coming of Christ, and rejoices in being “the friend of the bridegroom”, whom he points out as “the Lamb of God, who takes away the sin of the world”.198 Going before Jesus “in the spirit and power of Elijah”, John bears witness to Christ in his preaching, by his Baptism of conversion, and through his martyrdom. 199
524 When the Church celebrates the liturgy of Advent each year, she makes present this ancient expectancy of the Messiah, for by sharing in the long preparation for the Savior’s first coming, the faithful renew their ardent desire for his second coming. 200 By celebrating the precursor’s birth and martyrdom, the Church unites herself to his desire: “He must increase, but I must decrease.“201
-Catechism of the Catholic Church, 2nd Edition, pp. 131-132