Detail from the Creation of Adam, Michelangelo, c. 1508–1512
ONCE one understands the Cross—that we are not mere observers but active participants in the salvation of the world—it changes everything. Because now, by uniting the whole of your activity to Jesus, you yourself become a “living sacrifice” who is “hidden” in Christ. You become a real instrument of grace through the merits of Christ’s Cross and a participant in His divine “office” through His Resurrection.
For you have died, and your life is hidden with Christ in God. (Col 3:3)
All this is another way of saying that you are now a part of Christ, a literal member of His mystical body through Baptism, and not just a mere “instrument” like a pipeline or tool. Rather, dear Christian, this is what happens when the priest anoints your brow with the chrism oil:
…the faithful, who by Baptism are incorporated into Christ and integrated into the People of God, are made sharers in their particular way in the priestly, prophetic, and kingly office of Christ, and have their own part to play in the mission of the whole Christian people in the Church and in the World. —Catechism of the Catholic Church, n. 897
THE KINGLY OFFICE
Through Baptism, God has “nailed” your sin and old nature to the wood of the Cross, and infused you with the Holy Trinity, thus inaugurating the resurrection of your “true self.”
We who were baptized into Christ Jesus were baptized into his death… If, then, we have died with Christ, we believe that we shall also live with him. (Rom 6:3, 8)
This is all to say that Baptism makes you capable of loving as God loves, and living as He lives. But this demands the ongoing renunciation of sin and the “old self.” And that is how you participate in the kingly office of Jesus: by becoming, with the help of the Holy Spirit, a “sovereign” over your body and its passions.
By virtue of their kingly mission, lay people have the power to uproot the rule of sin within themselves and in the world, by their self-denial and holiness of life… What, indeed, is as royal for a soul as to govern the body in obedience to God? —CCC, n. 786
This obedience to God means also subjecting yourself, as Christ did, to becoming the servant of others. ‘For the Christian, “to reign is to serve him.”‘ 
THE PROPHETIC OFFICE
Through Baptism, you have been drawn into, and so deeply identified with Jesus, that what He did on earth He intends to continue to do through you—not as a mere passive conduit—but truly as His body. Do you understand this, dear friend? You are His body. What Jesus does and wants to do is through “His body”, just as what you need to do today is done through the activity of your mind, mouth, and limbs. How Jesus works through you and I will be different, because there are many members in the body.  But what is Christ’s is now yours; His power and dominion is your “birthright”:
Behold, I have given you the power ‘to tread upon serpents’ and scorpions and upon the full force of the enemy and nothing will harm you… Amen, amen, I say to you, whoever believes in me will do the works that I do, and will do greater ones than these, because I am going to the Father… (Luke 10:19; John 14:12)
Preeminent in the works of Christ is His mission to proclaim the Kingdom of God.  And thus,
Lay people also fulfill their prophetic mission by evangelization, “that is, the proclamation of Christ by word and the testimony of life.” —CCC, n. 905
So we are ambassadors for Christ, as if God were appealing through us. (2 Cor 5:20)
But even more profound than this participation in the kingly and prophetic ministry of Jesus is the participation in His priestly office. Because it was precisely in this office, as both high priest and sacrifice, that Jesus reconciled the world to the Father. But now that you are a member of His Body, you too share in his royal priesthood and this work of reconciliation; you too share in the ability to fill up “what is lacking in the afflictions of Christ.”  How?
I urge you therefore, brothers, by the mercies of God, to offer your bodies as a living sacrifice, holy and pleasing to God, your spiritual worship. (Romans 12:1)
Your every thought, word, and deed, when united to the Lord in love, can become a means by which the saving grace of the Cross is drawn into your soul, and upon others.
For all their works, prayers, and apostolic undertakings, family and married life, daily work, relaxation of mind and body, if they are accomplished in the Spirit—indeed even the hardships of life if patiently born—all these become spiritual sacrifices acceptable to God through Jesus Christ. —CCC, n. 901
Here again, when we “offer up” these works, prayers, and sufferings—as did Jesus—they take on a redemptive power that flows directly from the rent heart of the Redeemer.
…the weaknesses of all human sufferings are capable of being infused with the same power of God manifested in Christ’s Cross… so that every form of suffering, given fresh life by the power of this Cross, should become no longer the weakness of man but the power of God. —ST. JOHN PAUL II, Salvifici Doloros, n. 23, 26
For our part—in order for our spiritual priesthood to be effective—it calls for the obedience of faith. Our Lady is the model of the Church’s spiritual priesthood, for she was the first to offer herself as a living sacrifice in order that Jesus may be given to the world. No matter what we encounter in life, good and bad, the prayer of the priestly Christian should be the same:
Behold, I am the handmaid of the Lord. May it be done to me according to your word. (Luke 1:38)
In this way, the infusion of grace in all our acts transforms them, as it were, like the “bread and wine” are transformed into the Body and Blood of Christ. Suddenly, what from a human standpoint seem like meaningless acts or senseless sufferings become ‘“a fragrant aroma,” an acceptable sacrifice, pleasing to God.’  For, when freely united to the Lord, Jesus Himself enters into our works such that “I live, no longer I, but Christ lives in me.”  What effects the “transubstantiation” of our acts into something “holy and pleasing to God” is love.
So be imitators of God, as beloved children, and live in love, as Christ loved us and handed himself over for us as a sacrificial offering to God for a fragrant aroma… and, like living stones, let yourselves be built into a spiritual house to be a holy priesthood to offer spiritual sacrifices acceptable to God through Jesus Christ.(Eph 5:1-2,1 Peter 2:5)
LOVE CONQUERS ALL
Dear brothers and sisters, let me indeed reduce this teaching to one word: love. It is that simple. “Love, and do what you will,” Augustine once said.  That is because the one who loves as Christ loved us will always be participating in His kingly, prophetic, and priestly office.
Put on then, as God’s chosen ones, holy and beloved, heartfelt compassion, kindness, humility, gentleness, and patience, bearing with one another and forgiving one another, if one has a grievance against another; as the Lord has forgiven you, so must you also do. And over all these put on love, that is, the bond of perfection. And let the peace of Christ control your hearts, the peace into which you were also called in one body. And be thankful. Let the word of Christ dwell in you richly, as in all wisdom you teach and admonish one another, singing psalms, hymns, and spiritual songs with gratitude in your hearts to God. And whatever you do, in word or in deed, do everything in the name of the Lord Jesus, giving thanks to God the Father through him. (Col 3:12-17)