Entering Jerusalem, Jesus is fulfilling His exodus to His Father, but He means to take us all back with Him. He aims to transfigure by the glory of divine love this earthly City that will torture Him. But "what is the city but the people?" The New Jerusalem can only be realized in the sanctification of each of her citizens.
The matins readings for Palm Sunday show this. From Hebrews 10:14, we have, "By one offering, [Jesus] has forever perfected those who are being sanctified."
Within the one, all-sufficing, sacrifice of Jesus, the justification of humanity, and of each human, IS accomplished ("perfected," teteleioken). But that metaphysical, once-for-all fact contains within it the whole drama of personal development ("being sanctified," hagiazomenous). The perfecting of a creature takes time. It occurs through suffering, as Hebrews also says. The history of suffering must become a drama of solidarity in Christ, the drama of vicarious substitution within the medium of Christ's consubstantial solidarity with all of humanity.
Here is Saint Andrew of Crete from a Palm Sunday sermon: "In His humility, Christ entered the dark regions of our fallen world, and He is glad that He became so humble for our sake, glad that He came and lived among us and shared in our nature in order to raise us up again to Himself. And even though we are told that He has now ascended above the highest heavens,...His love for man will never rest until He has raised our earthbound nature from glory to glory, and made it one with His own in heaven."
Dogmatically speaking, we are justified by baptism into Christ Jesus, and then the whole life of prayer, sacraments, and morality is meant to INCREASE that justification. Increasing justification is called sanctification. The culmination of sanctification is the full intimacy of participation in the Trinitarian communion that is our deification (beatific vision, beatific loving).
There is often a difference between Protestant and Catholic on this point. On the one hand, the powerful emphasis on justification in the communities of the Reformation is something Catholics need to appreciate more profoundly. On the other hand, it is characteristic of Catholicism to emphasize the fact that justification must increase.
What happens when we are baptized into the Paschal Mystery? We are justified. What does that mean? It means that we are, ontically (at the level of being), brought into friendship with the Trinity. But friendships must grow, or they die. Growing in our intimacy with God causes our sanctification and is the fruit of our sanctification.
Let's try again. By dying out of love for every single human being, Jesus has justified every single human being. There is a bright and fine idea in the Logos of the Father, from before the foundation of the world, of each of us. He has drawn an icon of each of us. Our best and truest self is the saint the Father dreams us to be. That idea is perfect. But no creature can start off perfect, for creatures come from nothing, and have being not from themselves but from God. If the trajectory of the created person is to go from nothing (creation ex nihilo) to everything (participation in God as ipsum esse subsistens), that takes time and history. (Christ's humanity and Mary are exceptions that prove the rule.)
If in time and history, the vicissitudes of finitude have been incalculably compounded by the immense entropies of fallenness, due to our consubstantial solidarity, then when God the Father creates each of us at our conception, His dream of us has a very long way to go indeed if it is to be realized in flesh and blood.
When Christ dies for all of us, the Father is definitively saying His Yes to each of us: it is good that you exist. It is very good. That's essential to the Gospel. That's the way we need to look at each other, Christian and non-Christian: the way the Father looks at us. No evangelization without that gaze of existentially justifying love.
What baptism does is make that existential affirmation, which has already been worked as a fact by Jesus in His Paschal Mystery, a living reality in our lives. When we sin, we deface that icon the Father has always drawn of us. We become wretched parodies of our true selves. If we learn that the Father really does love us, what good is that, if I have no way to leave my self-lacerating and self-marring habits behind? The Christian life of prayer, sacraments, and morality is that exodus out of our antic self-parody.
By baptism, Jesus has us, and by His Spirit, can begin the process of fashioning us according to His Father's dream of us. As we relinquish our own ideas of who we are, we become more and more who God know us to be. We begin to shine with the glory of divine love. And a new city begins to rise.