Of the four Advent Sunday readings this year, I was struck that each first reading features a passage from the Book of Isaiah. As we shall see, this is not a coincidence. The Gospels also present us with several different accounts of John the Baptist invoking Isaiah, for the purpose of arguing at Jesus fulfills the prophecy made by the venerable Old Testament prophet (John 1:23-27, Mat. 3:1-3, Mark 1:1-8, Luke 3:1-6).
The passage that John the Baptist quotes in each of the four Gospels has its origins in Isaiah 40:1-5:
Speak tenderly to Jerusalem, and cry to her that her warfare is ended, that her iniquity is pardoned, that she has received from the LORD’s hand double for all her sins. A voice cries: “In the wilderness prepare the way of the LORD, make straight in the desert a highway for our God. Every valley shall be lifted up, and every mountain and hill be made low; the uneven ground shall become level, and the rough places a plain. And the glory of the LORD shall be revealed, and all flesh shall see it together, for the mouth of the LORD has spoken.”
There is a lot of explicit Christology in this passage. Isaiah predicts that at some time in the future, the glory of the Lord shall be revealed, and all flesh shall see it. He is, of course, speaking of God-become-man, though Isaiah would not have known it at the time. His next phrase, the mouth of the Lord has spoken (Is 40:5) is itself a prophetic reference to the Word, who is God incarnate, Jesus Christ.
Isaiah is an Old Testament herald of the Messiah. For that reason, his testimony is often part of the Advent cycle of readings. By the same token, John the Baptist completes the line of Old Testament prophets. He is not only the “precursor” or “forerunner” of Jesus, but he also happens to be the cousin of the Messiah.
In his commentary, “Isaiah,” The biblical scholar Brevard Childs argues that the student of Scripture is not amiss when he or she makes the theological connection between the writing of Isaiah and the speech of John the Baptist. John announces to his followers to “prepare the way of the Lord” (cf Is. 40:3). As John preaches in the wilderness, he is rather self-consciously alluding to Isaiah.
Since all four Evangelists connect John the Baptist with the passage in Isaiah, it is fair to conclude that the Evangelists themselves see the fulfillment of Isaiah’s Messianic prophecy as being immanent. At the same time, we can see the completion of the prophetic cycle, beginning with Isaiah and finally ending with John, as to the coming of the Messiah in the person of the baby Jesus. The Old Testament prophet anticipates the coming of the Lord, and the New Testament prophet confirms the realization of the prophecy, and in so doing, validates the teaching of the Old Testament, and the faith of its adherents.
If you think I am taking this parallel too far, then consider this Advent homily to college students, prepared by John Paul II eight years ago. Here is the Gospel reading for the Second Sunday in Advent:
John the Baptist appeared, preaching in the desert of Judea
and saying, “Repent, for the kingdom of heaven is at hand!”
It was of him that the prophet Isaiah had spoken when he said:
A voice of one crying out in the desert,
Prepare the way of the Lord,
make straight his paths.
John wore clothing made of camel’s hair
and had a leather belt around his waist.
His food was locusts and wild honey.
At that time Jerusalem, all Judea,
and the whole region around the Jordan
were going out to him
and were being baptized by him in the Jordan River
as they acknowledged their sins.
When he saw many of the Pharisees and Sadducees
coming to his baptism, he said to them, “You brood of vipers!
Who warned you to flee from the coming wrath?
Produce good fruit as evidence of your repentance.
And do not presume to say to yourselves,
‘We have Abraham as our father.’
For I tell you,
God can raise up children to Abraham from these stones.
Even now the ax lies at the root of the trees.
Therefore every tree that does not bear good fruit
will be cut down and thrown into the fire.
I am baptizing you with water, for repentance,
but the one who is coming after me is mightier than I.
I am not worthy to carry his sandals.
He will baptize you with the Holy Spirit and fire.
His winnowing fan is in his hand.
He will clear his threshing floor
and gather his wheat into his barn,
but the chaff he will burn with unquenchable fire.”
And here is Pope Benedict’s reflection on the Gospels, as summarized in his Sunday Angelus of 2007:
Yesterday, the Solemnity of the Immaculate Conception, the liturgy invited us to turn our gaze to Mary, Mother of Jesus and our Mother, Star of Hope for every person. Today, the Second Sunday of Advent, it presents to us the austere figure of the Precursor, whom the Evangelist Matthew introduces as follows: “In those days came John the Baptist, preaching in the wilderness of Judea: “Repent, for the Kingdom of heaven is at hand'” (Mt 3: 1-2). His mission was to prepare and clear the way for the Lord, calling the people of Israel to repent of their sins and to correct every injustice. John the Baptist, with demanding words, announced the imminent judgement: “Every tree, therefore, that does not bear good fruit is cut down and thrown into the fire” (Mt 3: 10). Above all, John put people on guard against the hypocrisy of those who felt safe merely because they belonged to the Chosen People: in God’s eyes, he said, no one has reason to boast but must bear “fruit that befits repentance”.
While the Advent journey continues, while we prepare to celebrate the Birth of Christ, John the Baptist’s appeal for conversion rings out in our communities. It is a pressing invitation to open our hearts to receive the Son of God, who comes among us to make manifest the divine judgement. The Father, writes John the Evangelist, judges no one but has given all judgement to the Son because he is the Son of Man (cf. Jn 5: 22, 27). And it is today, in the present, that our future destiny is being played out. It is our actual conduct in this life that decides our eternal fate. At the end of our days on earth, at the moment of death, we will be evaluated on the basis of our likeness – or lack of it – to the Child who is about to be born in the poor grotto of Bethlehem, because he is the criterion of the measure that God has given to humanity. The Heavenly Father, who expressed his merciful love to us through the birth of his Only-Begotten Son, calls us to follow in his footsteps, making our existence, as he did, a gift of love. And the fruit of love is that fruit which “befits repentance”, to which John the Baptist refers while he addresses cutting words to the Pharisees and Sadduccees among the crowds who had come for Baptism.
Through the Gospel, John the Baptist continues to speak down the centuries to every generation. His clear, harsh words are particularly salutary for us, men and women of our time, in which the way of living and perceiving Christmas unfortunately all too often suffers the effects of a materialistic mindset. The “voice” of the great prophet asks us to prepare the way of the Lord, who comes in the external and internal wildernesses of today, thirsting for the living water that is Christ. May the Virgin Mary guide us to true conversion of heart, so that we may make the necessary choices to harmonize our mentalities with the Gospel.
 Isaiah. Brevard Childs. p303