When we say this truth of our faith, we are proclaiming to the world that we acknowledge the characteristics by which the Church founded by Christ can be recognized. That sounds pretty important, so let's take a look at each one.

• One - this describes not only the unity but the uniqueness of the Church, a characteristic that something founded by God would certainly have. Having many churches (i.e. the Orthodox Christian Churches, also known as sui iuris churches) and ecclesial communities (which is the formal name for Protestant and non-denominational congregations) is not part of Christ's will, as he specifically says in John chapter 10 verse 16. Unity doesn't mean uniformity, but it does demand a unity of faith and heart. This is demonstrated by obedience of faith to the Magisterium of the Church.
• Holy - as the instrument left on Earth to further the life and work of Jesus, our Lord also imparted to it his divine holiness. This is extended to its members specifically through the sacraments and liturgy, which offer people a path to holiness through sharing in God's life. It is important to stress that the characteristic of holiness doesn't mean that members of the Church on Earth are perfect or that its ministers are incapable of sin. This is important to realize because it is only through such imperfections that the human elements of the Church are called to extend mercy - a church run by angels would very likely find such action difficult, if not impossible. It by precisely by sharing in the complete experience of human existence and despite the human defects of its members, the Holy Spirit continues to impart grace and mercy, keeping the mission of the Church true as the guidepost to the Beatific Vision. Part of this mission is the challenge of presenting the truth in a way that remains relevant to the changing understanding of the world (although never changing those truths themselves!)
• catholic - If you read the text of the Creed, this is a deliberate use of small letter "c" because it draws from the Greek word meaning "universal". The Church is meant to extend to the entire world, to be open to all of humanity without exception. If freely accepted, the doctrines of the Church not only save individuals; they also raise, strength and purify cultures. This mark is closely connected with unity: the Church would not be catholic if she did not draw all into one while still recognizing the variety and richness of the various cultures. As Vatican II noted, "In virtue of this catholicity each part contributes its own gifts to other parts and to the whole church, so that the whole and each of the parts are strengthened by the common sharing of all things and by the common effort to attain to fullness in unity" (Lumen Gentium 13).
• Apostolic - "Built upon the foundation of the apostles" (Ephesians 2:20), the Church hands on that faith. She does so in an unbroken line of succession using the same methods those first bishops did: through evangelization, catechesis and sanctification under the doctrine and authority of Christ. The bishops are the successors of the Apostles and the guardians of the faith in union with the successor of St. Peter, the Pope who is the Bishop of Rome In this regard they are aided by priests, who are their coworkers "for the proper fulfillment of the apostolic mission that had been entrusted to it by Christ." (Catechism of the Catholic Church 1562).

These marks are the tests that identify the Church Jesus wished to establish. In a certain sense too, they can be used as tests whether individual Christians are properly and fully connected to the "vine" that is the mystical Body of Christ. If one loves the unity of the Church and witnesses to protect it, if one live with a heart open with love to all who are created in the image and likeness of God, if one trusts in the links that bind us to Jesus - found as the hierarchy, the Magisterium and Apostolic Tradition in the Church - that person is open to living a life that moves them to holiness.

---