EWTN Catholic Q&A
Re: why are they not Roman Catholic
Question from confused on 06-08-2017:

I understand that "The Church after Pentecost was spread by the Apostles throughout the world, in different cultures, languages, and customs". I don't understand that "These apostolic Churches are not Roman nor should they be expected to conform to the Faith as expressed in Roman Catholicism". Isn't there only one, holy, catholic and apostolic Church?

Answer by Robert Klesko on 06-09-2017:

I would suggest reading from the Catechism of the Catholic Church #813-814. These paragraphs enumerate the various points of unity in the Church (foundation in Christ, one common baptism, etc.), while also demonstrating (in #814) that the one Faith is expressed in a diversity of Apostolic traditions. Included in these traditions are the Eastern Churches. The diversity of traditions does not diminish the oneness of the Faith.

Hope this helps!