What does it mean to confess a belief in one holy catholic church?

I like this from Mike Horton:

Scripture’s focus is on what God is doing rather than on what we are doing. The Triune God is saving sinners through preaching and sacrament. There is “one holy catholic and apostolic church” not because individual believers realized that they could more effectively reach the world and accomplish their goals in tandem. Rather, this church exists because of the Father’s eternal election of a people, the Son’s mediation and saving work for them, and the Spirit’s work of uniting them to Christ through the gospel. We are recipients of a kingdom; the Father is the builder, by his Son and Spirit, through the Word.

Therefore, there really is one church—catholic, spread throughout the world yet united in one Lord, one faith, one baptism—even though its visible shape right now seems to speak against it. Same thing with the holiness of the church: holy in Christ, it is nevertheless “simultaneously justified and sinful.”

This quote comes from a blog post "Have Denominations Had Their Day?" that's well worth reading.