Like the whole liturgy of this season, the purpose of the Mass for the Fourth Sunday of Advent is to prepare us for the twofold coming of Christ: His coming in mercy at Christmas; and in justice at the end of the world.
In this mass we meet once again with the three great figures that are before the mind of the Church throughout Advent: Isaias, St. John the Baptist, and our Lady. The prophet Isaias foretells of St. John the Baptist that he will be "A voice of one crying in the wilderness: Prepare ye the way of the Lord, make straight his paths ... and all flesh shall see the salvation of God." And "the word of the Lord was made unto John, the son of Zachary in the desert. And he came into all the country about the Jordan, preaching the Baptism of penance for the remission of sins" (Gospel).
St. Gregory explains,
John told those who hurried in crowds to be baptized: ‘Ye brood of vipers, who hath told you to flee from the wrath to come?’ Now the wrath to come is the final chastisement, which the sinner will not be able to escape unless he have recourse now to the lamentations of penance. The friend of the Bridegroom warns us to bring forth not fruits merely of penance but worthy fruits. These words are a call to each man's conscience, bidding him to lay up by means of penance a treasure of good works, the greater in proportions to the ravage of sin which caused it.
And St. Leo says, "God Himself teaches us by the prophet Isaias: I will lead the blind in a way that they know not, and I will turn the darkness before them into light and I will not forsake them."
Come, Lord Jesus, and tarry not.