In the WSJ, Frederica Mathewes-Green explains the differences between the RC and Orthodox understandings of "unity."

From a Roman Catholic perspective, unity is created by the institution of the church. Within that unity there can be diversity; not everyone agrees with official teaching, some very loudly. What holds things together is membership. This kind of unity makes immediate sense to Americans: Whatever their disagreements, everyone salutes the flag, and all Catholics salute, if not technically obey, Rome’s magisterium.

When Roman Catholics look at Orthodoxy, they don’t see a centralized, global institution. Instead, the church appears to be a jumble of national and ethnic bodies (a situation even more confused in the U.S. as a result of immigration). To Catholics, the Orthodox Church looks like chaos.

But from an Orthodox perspective, unity is created by believing the same things.

What the Catechism of the Catholic Church says about being "one" as in "one, holy and apostolic"

815 What are these bonds of unity? Above all, charity "binds everything together in perfect harmony."²⁶⁵ But the unity of the pilgrim Church is also assured by visible bonds of communion:

– profession of one faith received from the Apostles;

-common celebration of divine worship, especially of the sacraments;

– apostolic succession through the sacrament of Holy Orders, maintaining the fraternal concord of God’s family.²⁶⁶