It is Christ who, through the Holy Spirit, makes his Church one, holy, catholic, and apostolic, and it is he who calls her to realize each of these qualities.

Catechism of the Catholic Church #811

The Four Marks of the Church

1. One - Unity
2. Holy - Sanctity
3. Catholic - Universality
4. Apostolic - Continuity

One

The pope is the supreme head of the Church who makes sure that the same doctrine that has been taught since the time of the original Apostles continues to be taught correctly today. There has never been a change in Church doctrine (there have been changes in Church practices, such as language of liturgy, celibacy of the priesthood, etc., but the essential Truths have always been consistent).

The pope also ensures that the celebration of the liturgy and sacraments is the same for all Catholics. Throughout the world bread and wine are used for the Eucharist, water for Baptism, oil for Confirmation, and similar signs for each of the seven sacraments. We may have different cultures, but we have a unified Church.

The pope is not alone with all this authority, of course. He is guided by the Holy Spirit who dwells in all believers. The unity we have enjoyed for 2000+ years is thanks to having God in our midst.

Unfortunately, human sinfulness has threatened our unity for the same 2000+ years. The Apostles dealt with disagreements, rifts, heresies. The same threats to unity have continued over the centuries, with schisms, reformations, and more heresies. It is no different today.

Over the past fifty years Catholics have been called to ecumenism - the reconciliation of all Christians back into a unified fold. We are encouraged to pray with members of different faiths, celebrate what we have in common, and collaborate in being of service to others. Unifying the divisions will take joint effort, plenty of prayer and, of course, continued divine intervention.

Holy

Our personal holiness is imperfect, but we are still called to live holy lives. We can passionately practice our faith by participating in the celebration of the liturgy and the sacraments. We can pray, read scripture, and continually nurture our faith. We can share our faith with the rest of our family, the Mystical Body of Christ. Together we can live a holy life.

Although we are all called to be saints, there are some people whose lives have been renowned for their holiness. To read a commentary on saints and sainthood, click here.

Catholic

The Church is universal in time, from the very first Pentecost. Peter and the Apostles, filled with the Holy Spirit, spread the Good News to all who could hear. Despite language barriers, all understood. Jesus is with the Church; He sent the Advocate to guide the Church for all time. The Church has been there to teach and preach to the whole human race since the very beginning.

The Church is universal in place. It is not localized to one town or country. It is in every country, every continent. Each individual parish is part of the whole Church, unified in the common goal of being universal.

The Church is universal in doctrine. It teaches the same doctrine and administer the same Sacraments everywhere, to everyone. All people, rich and poor, sinners, educated and uneducated, at all phases of the faith journey, share the teachings and Sacraments of the Church.

The Church is so universal even non-Catholics and non-Christians belong to it!. The Catechism of the Catholic Church states, "All men are called to this catholic unity of the People of God...And to it, in different ways, belong or are ordered: the Catholic faithful, others who believe in Christ, and finally all mankind, called by God's grace to salvation." (#836) All salvation comes from Christ through His Universal Church.

Apostolic

The Church and its teachings can be traced directly back to Jesus and the apostles who He appointed as His successors. Simon Peter became the Rock, the head of the Church, after the Resurrection. Every pope has been a successor to Peter,  (Benedict XVI is not the successor of John Paul II; he is the successor of St. Peter). The Sacrament of Holy Orders is the direct link for every ordained deacon, priest, and bishop to the original twelve apostles.

The appointment of Peter as the Rock, the foundation of our Church, takes on an even more powerful meaning when reading Psalm 95, which begins, "Come, let us sing to the Lord and shout with joy to the Rock who saves us." The psalm is one of praise to God, our heavenly Father. Jesus appointed Peter, and his successors, our Church fathers, who have been called to lead us on our spiritual journeys. The Catholic Church, based on both Sacred Tradition and Sacred Scripture, is truly apostolic.