In the Nicene Creed, Christians all around the world confess that there is "One Holy Catholic and Apostolic Church."
Today, I want to focus on the meaning of "One."
Almost every scholar in the history of the church agrees on "ONE" thing... that as used the creed, the "ONE" catholic church has two senses:
1. The Universal Church - The "ONE" church is a Universal Church (catholic means "universal"). The Universal Church is made up of Christians who confess the basic doctrines of Nicene Creed about the Trinity and about Christ and His nature and work. In other words, Christians in lots of places disagree about many things, but they are still a part of the "ONE" Holy, Catholic and Apostolic Church.
2. There are local expressions of the "ONE." In other words, there are "MANY" churches that make up the "ONE" church. In the New Testament, the "ONE" Body has "MANY" parts. Many, but "ONE."
There are some churches (such as the various Catholic Churches (Orthodox, Coptic and Roman Catholic), who claim that they are the ONLY "ONE" church. In other words, you have to be part of them in order to be part of the "ONE" (as in "true" church).
Protestant churches disagree. Most protestant churches believe that there is one UNIVERSAL church and then many individual churches. These churches may belong to denominations, but essentially, they are separate churches. Furthermore, most Protestants believe that Catholic churches are not true churches.
It's a mess.
If we stick to the creed (the Nicene Creed), as Christians, we can simply confess what the creed says... there is one, holy catholic and apostolic church. What is clear historically is that the writers of the creed believed that the "ONE" church consisted of all churches that agreed with the basic tenants (ideas) of the Nicene Creed. The creed says nothing about Rome, Popes, no Popes or specific styles of worship. To assume more is to go outside of the creed.
What we can all agree upon is that there is ONE, holy catholic (small "c") church. ONE Universal Church, which is both Holy and Apostolic.