From "The Spirit of the Church in the Course of the Christian Year":
TThis time is intended to prepare for the Coming and Birth of Jesus Christ. Although the Lord must no longer be born outwardly, as he must no longer die, he nevertheless renews in a spiritual and interior way in his Church, and in each Faithful in particular, the Mysteries that he once fulfilled to save us; and to each Mystery is attached a grace that relates to it, that reminds us of its presence, applies its virtues to us, and makes us draw the fruit.
The four Weeks that make up Advent, represent to us the four thousand years, which, since the origin of the world, had preceded the Birth of Jesus Christ. God, before sending his Son to earth, had wanted men to feel for a long time the deep misery in which sin had plunged them; that this feeling might humble them, and excite them to recognize and desire the one Liberator, who could break their chains, and heal their ills.
Blessed, then, are those who in this holy time will feel how miserable, weak, languishing they are, dominated by their senses and their passions; unable by themselves to do good, and inclined to any kind of evil. This feeling is the first step towards their deliverance and healing!
The Patriarchs represented the Messiah. The Prophets predicted and announced it. All the righteous desired and demanded it. The Ungodly forgot it, and walked away from it. John the Baptist pointed to it to make them recognize it. Let us be among the faithful, and let us trace by the practices of true piety, as be appropriate to the Law of Grace, what was done in that happy time, to serve as preparation for the Birth of the Redeemer. Above all, let us be continually occupied with the desire to see It born spiritually in our hearts, and reign in our souls; and let us use, in order to attract this Birth and to establish this Kingdom, the expressions of the Prophets, which the Church uses for her Prayers in these holy days. O Heavens! send from above your dew to the earth, and you, clouds, open your breast, and rain down the Righteous. O eternal Wisdom! come and enlighten me! O Key of David, come and open to me the door that sin had closed for me! O King of the Nations, come and bring me salvation! etc.
The Gospel of the first Sunday of Advent describes to us the last Advent of Jesus Christ at the end of the centuries, which will be an Advent of justice and rigor, to commit us to enjoy the first one that shows only gentleness and mercy.
The Gospel of the second Sunday represents St. John the Baptist, acting as the Precursor of the Messiah, from the very middle of the prison where he is held, sending his Disciples to Jesus Christ.
The Gospel of the Third Sunday offers us the solemn deputation that the Jews made to St. John, to know if he was not himself the Messiah; and St. John sending them back to the one who alone deserved this divine quality. At the same time he makes this just reproach, that this Messiah, so desired, was in their midst, and that they did not know him.
The Gospel of the Fourth Sunday makes us see this Blessed Precursor, raising his voice on the banks of the Jordan, and in all the neighboring countries, to proclaim the Baptism of Penance, which served as preparation for the Baptism of the New Law, which Jesus Christ was to institute, and saying with redoubled cries; Make God's paths straight and united, for all flesh will see God's sent salvation.
Finally, on Christmas Eve, we are repeated several times these consoling words; Tomorrow the iniquity of the earth will be erased, and the Savior of the world will reign over us.