In the Niceno-Constantinoplitan Creed (henceforth, the Nicene Creed, or NC, because Niceno-Constantinopolitan is ridiculously long and nobody says that anyway), we confess our faith in 'one holy, catholic and apostolic church'. This phrase constitutes a problem in our day, when the church is splintered and divided, and where there are competing interpretations as to what it means to be holy, catholic and apostolic. In particular, does holiness necessarily involve some particular experience of the Holy Spirit? Does catholicity necessarily involve communion with the bishop of Rome? Does apostolicity (is that a word?) require the presence of living apostles?
Where is the church?
I think Jesus' prayer in John 17 goes a long way to answering all these questions. The prayer is a natural place to look when we're thinking about the church. Here, the church's Lord prays for the future of his called and assembled people. And, I dare say not coincidentally, the three marks of the church in NC are found right here.
In NC, we confess the church to be 'holy'. In John 17:17, Jesus prays for his disciples: "sanctify them in the truth; your word is truth". Set them apart, make them holy. Verse 14 contributes the fact that in giving the disciples God's word, Jesus has transferred them out of the sphere of "the world" (though not out of the world itself) and that they are no longer "of the world". This is the very definition of holiness in Scripture: to be separated from the world and to God. And notice, the instrument by which this is done is God's word.
In NC, we acknowledge the church to be 'catholic'. In John 17:21, Jesus prays for all his disciples throughout all the ages, "that they may all be one". The unity he asks for is comparable to the unity between the Father and the Son - so, pretty close! The ground of their unity with each other is to be their unity with God and his Christ - "all mine are yours, and yours are mine" (v. 10), "just as you Father are in me and I in you, that they also may be in us" (v. 21) - and this again occurs through the instrumentality of the word (v.8, v.20).
In NC, we confess our faith in a church that is 'apostolic'. In John 17:20, Jesus prays for "those who will believe in me through their word" - i.e. the message carried by the apostles. Previously, he has prayed for the original disciples as those who are sent out into the world just as he was sent (v. 18). I take it that they are being sent to take the word of God, just as Jesus was. The church is to be founded on their word.
A couple of notes on this. Firstly, the church is holy, catholic and apostolic wherever it is ruled and governed by the word of God, and where it recognises in this word of God its very life and only source of being. Wherever we see a congregation that lives by the word of God, there we see the one holy, catholic and apostolic church confessed in the Nicene Creed. In concreto, this will mean that wherever we see a group of people bound together by the fact that they are also bound to the Holy Scriptures, we see the church. Those who are bound to the apostolic message in Scripture are also separated from the world and united to one another, because it is the word of God that gives them life. There are no other criteria for a true church.
Secondly, this is an article of faith. We believe that this church exists amid the dissension and rivalries that exist in the churches. We believe it because the Father always hears the Son, and the Son prayed for this.