Be strong, fear not! Here is your God, he comes with vindication; with divine recompense; he comes to save you.
The readings speak of good news: evanggelion; a Greek word that means extraordinarily good news, such as the birth of a king, or a military victory. It was a word that was rarely used precisely because the birth of a king and military victories were not things that happen every day. The readings for Advent announce the good news, the birth of a king, and this king is born in order to bring about a military victory, a victory over sin and death.
Christianity is not a morality, it is not a system of ethics, it is not a philosophy - much less is it an ideology. It is, rather, a message of salvation. That's what distinguishes Christianity from every other world religion; it is a message of salvation. God saves. God has come to save us. Even the name "Jesus" is the late form of the Hebrew name Joshua which means "Yahweh is salvation".
One does not need "faith" to believe that God exists; the existence of God is demonstrable through human reason. To believe that God saves, however, does require the gift of faith. Human reason cannot demonstrate that God saves. That has been revealed in this reading from Isaiah: "Here is your God, he comes with vindication; with divine recompense, he comes to save you." Save you from what? From eternal death, which is spawned by a life of sin. We do not have the capacity to rise above our sinfulness. We will sin, and scripture points out that the wages of sin is death. Sin is rotten, but we have very little sense of the seriousness of sin. I know I don't have the sense of the seriousness of sin that I ought to have. Think of the parable of the wedding banquet (Mt 22, 1-10); the king was enraged. Why would he be enraged? Anger is a response to a perceived injustice, and the anger in this case was the result of the serious sin of rejecting the divine offer of salvation: I have better things to do, I have business to take care of, more important matters, etc.
This world is passing away, our lives are passing away; our life is short. This life is about preparing for eternity, and we have been invited, as sheer gift, to partake in the king's banquet for his son, and many people, in a spirit of tremendous arrogance, will tell God that they have more important things to attend to. The result is that the king is enraged.
The first beatitude is "Blessed are the poor in spirit, the kingdom of heaven is theirs". The poor in spirit are those who recognize their utter need for God; they know their own interior poverty. They recognize the tremendous gift that is offered them, and so they open themselves to it. The kingdom of heaven is theirs. But sin blinds us to our poverty. And here is the danger of riches. When we have so much money, life is good, we don't struggle, we don't see our own poverty of spirit. That is why we are at our worst in times of prosperity, but we are at our best in times of suffering; and that's why Christ said that it is difficult for a rich man to enter into the kingdom of heaven. When all is well, who needs God? And that's when the practice of our faith becomes sporadic - whenever we can, whenever there is time; there are more important matters to be taken care of first; then and only then will we see about partaking in the sacrificial banquet of the Mass, the Eucharist.
But the Eucharist is that military victory over sin and death. The Eucharist is the sacrifice of Calvary. It is Good Friday, contained under the appearance of an ordinary piece of bread. When we partake of that sacrifice, we are fed with the bread of life, and the grace of God that alone enables us to rise above our sinfulness is given to us. We are saved from ourselves; our blindness is slowly dispelled. Our minds are enlightened by that grace. We begin to see the world with eyes that are not sick, but healthy. And so we begin to see reality as it is; we become more grateful, and greater gratitude translates into a deeper joy.
But many people would rather live in darkness. The reason is that the prospect of acknowledging sin is too ominous. The idea of looking deep within oneself and making an inventory of the things we've done in the past that have brought suffering to others is just too painful. But that is the only Arc de Triomphe through which we enter onto the road of victory, through which we taste the joy of that good news, that military victory that Christ had achieved over the kingdom of darkness. Christianity is about Christ, and Christ came to save. The gospel is a message of salvation. Only those who acknowledge the need for salvation, the need to be saved from themselves, from sin and death, will taste the joy of that victory.