The Doctor of Church who was not a Catholic

Little is known of the life of the medieval Armenian monk St. Gregory of Narek, other than that he was a dedicated monk who lived his entire adult life in a monastery situated in todays’ eastern Turkey.  Saint Gregory was never formally canonized. He was not a Catholic, though he did pertain to an apostolic church with legitimate sacraments and a hierarchical structure which, however, is not in formal communion with Rome.

During a Mass in 2015 commemorating the hundredth anniversary of the Armenian genocide by the Ottoman Turks, concelebrated a Mass at the Vatican with Patriarch Nerses Bedros XIX Tarmouni Pope Francis declared this saint of the Armenian Apostolic Church a Doctor of the Church, the second from the churches of the East, for his timeless and theologically sound writings. 

Armenian people is that their nation was the first to adopt Christianity as its official religion. Approximately twelve years before the Roman Emperor Constantine legalized Christianity in 313, an Armenian King converted to Christianity. Following the universal custom of mankind, the King’s religion then became his people’s. 

Gregory of Narek wrote mystical poetry, hymns, and biblical commentaries. He is one of Armenia’s greatest literary figures and poets. His principal work, the “Book of Lamentations,” consists of ninety-five prayers. He described it as “encyclopedia of prayer for all nations”. His other major works was "Commentary on the Song of Songs", which has been celebrated for its clarity and theology.

Note:

As opposed to the Armenian Catholic Church that began in the 17th century, the earlier Armenian Apostolic Church did not accept the Council of Chalcedon’s 451 teaching that Christ was fully divine and fully human. In 1996, St. John Paul II and Apostolic Catholicos Karekin I signed a declaration confirming the common faith of their two Churches.

References
https://mycatholic.life/saints/saints-of-the-liturgical-year/saint-gregory-of-narek/
https://www.britannica.com/biography/Saint-Gregory-Narekatzi
https://www.franciscanmedia.org/saint-of-the-day/saint-gregory-of-narek