The Writer’s Guide to Ecclesiastical Titles
Besides royalty and nobility, members of the clergy also have a rich history of titles and forms of address. Since religion, especially Catholicism and Eastern Orthodoxy, played such a large part in European society throughout much of history, it is important for a writer who is setting their story in this period or one drawing inspiration from it to know the basics of church hierarchy and forms of address.
If you are writing fantasy, they can be discarded completely in favor of your own inventions, if you so wish.
Pope and Patriarch
The pope is the head of the Roman Catholic Church and the patriarch is the leader of the Eastern Orthodox church.
The most common form of address for the pope is “your Holiness.” This title can also be used when referencing him, e.g., “his Holiness” or “His Holiness Pope Francis.” The term “Holy Father” is also used as well as “Most Blessed Father” and “Most Holy Father.” During the Middle Ages, the term “Dominus Apostolicus,” meaning “the Apostolic Lord,” was used.  Other titles have been applied to the pope, including Bishop of Rome, Vicar of Jesus Christ, Successor of Peter, and many more. 
The Ecumenical Patriarch of the Eastern Orthodox church is addressed as “Your All-Holiness” and referred to as “His All-Holiness.” The formal form of address is “Bartholomew, by the grace of God Archbishop of Constantinople, New Rome and Ecumenical Patriarch.” 
A cardinal oversees a large territory and directs the archbishops and bishops that report to him. Collectively, they make up the college of cardinals. One of their responsibilities is selecting the next pope after the previous one dies or retires. In the Catholic church, they are well known for their red vestments.
They are commonly addressed as “Your Eminence” and referred to as “His Eminence.” They are considered princes of the church and can be addressed in the same way with “Your Grace.” 
An archbishop leads an archdiocese, a large or heavily populated area that included all the churches located inside its boundaries.
They are addressed as “Your Excellency” and referred to as “His Grace Archbishop (Name).” Other common forms of address include “Your Grace” with the title being “The Most Reverend.”
Eastern Orthodox archbishops are referred to as “The Most Reverend Archbishop (Name) of (Place)” and addressed as either “Your Beatitude” or “Your Eminence.” 
A bishop oversees one or more dioceses and is under the authority of their archbishop.
The common form of address in the Roman Catholic church is “Your Grace.” He is referred to as “Bishop (Name).”
Bishops in the Eastern Orthodox church as referred to as “The Most Reverend Bishop (Name) of (Place)” and addressed as “Bishop (Name).”
A priest handles a parish church.
His is usually addressed and referred to as “Father (Name)” or just “Father.” An Eastern Orthodox priest is also called “Father” but can additionally be referred to as “The Reverend Father.” 
In both the Roman Catholic and Eastern Orthodox churches, there are religious orders whose members are referred to as monks, if male, and nuns, if female. In the Catholic tradition, they are called “Brother (Name)” and addressed as “sister” or “Sister (Name). It’s the same in the Eastern Orthodox tradition, although monks are referred to as “Father” and nuns as “Mother” or “Sister.”
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 Guruge, Anura (2008). Popes and the Tale of Their Names. AuthorHouse. ISBN 978-1-4343-8440-9.  Annuario Pontificio, published annually by Libreria Editrice Vaticana, p. 23*. ISBN of the 2012 edition: 978-88-209-8722-0.  Rodopoulos, Panteleimon (2007). "Institutions of the Ecumenical Patriarchate". An overview of Orthodox canon law. Translated by Lillie, W.J. Rollinsford, N.H.: Orthodox Research Institute. p. 213. ISBN 1-933275-15-4. OCLC 174964244.  https://en.wikipedia.org/wiki/List_of_religious_titles_and_styles#Catholicism  https://en.wikipedia.org/wiki/Archbishop