The simplest explanation of the apostolic nature of the true Church is in terms of apostolic succession. The bishops today and throughout history are true successors of the apostles because of the unbroken chain of consecration - the laying on of hands from one generation to the next.
To be sure, any sacrament has a material aspect that is essential to its validity. But one might say that this pared-down understanding of the apostolic nature of the Church takes the Holy Spirit for granted.
Isn't it presumptuous to assert that the apostolic faith is preserved and nurtured simply because of an unbroken physical chain of the laying-on of hands?
The Church's understanding of its apostolic nature is subtler than that. The apostles, says the Catechism of the Catholic Church, are "the witnesses chosen and sent on mission by Christ himself" (n. 857).
"Witnesses" is the key word here. The apostles were not packages of information. They had personally encountered Christ. Their knowledge of the man beside whom they had walked was transformed by the Paschal Mystery and the Holy Spirit.
The apostolic Church is the Church that encounters Christ in his fullness. It shares in the truth about Christ. This truth includes the truth of propositions such as Jesus is the only Son of God. It also includes the "truth" of encountering Christ in Scripture, in the sacraments, in prayer and in the shared life of the faithful.
"With the help of the Spirit dwelling in her," the Catechism continues, "the Church keeps and hands on the teaching, the 'good deposit,' the salutary words she has heard from the apostles.
"She continues to be taught, sanctified and guided by the apostles until Christ's return, through their successors in pastoral office: the college of bishops, assisted by priests, in union with the successor of Peter, the Church's supreme pastor" (n. 857).
The apostolic nature of the Church means being faithful to the past, but also fidelity to the future. Theologian Yves Congar wrote that apostolicity "makes the Church fill the space between the Alpha and the Omega by ensuring that there is continuity between the two and a substantial identity between the end and the beginning."
This is no small chore, not something to be taken for granted. Indeed, this task is one where no human or group of humans can hope to achieve success. No Spirit, no Church.
Involvement in the apostolic Church ought to make us humble, not proud. We have empty hands, nothing of our own to offer. Without an encounter with Jesus from one generation to the next, we are barren. To encounter Jesus and to witness to that encounter can only occur through the active presence of the Holy Spirit.
At the Last Supper, Jesus clearly spoke of the two missions - the mission of the apostles and the mission of the Spirit: "When the Advocate comes, whom I will send to you from the Father, the Spirit of truth who comes from the Father, he will testify on my behalf.
"You also are to testify because you have been with me from the beginning" (John 15.26-27).
For the mission of the apostles to be successful so must the mission of the Spirit. The opposite is also true: There is no successful mission of the Spirit without the apostolic mission.
There is order to the two missions. Spontaneous outpourings of the Spirit are crucial but so is the structured communion. If 50 years ago there was an overly legalistic conception of the Church, today there is also the contrary danger of an insistence on unstructured "spirituality."
Fidelity to the apostles is the fourth mark of the true Church. But it is only through the encounter with Christ, given to us by the Holy Spirit, that we can have an apostolic faith.
The Church waits, yearning for Christ's return. Thanks to the Holy Spirit, we wait dressed for action with our lamps lit (Luke 12.35).