Before addressing specific thinkers or traditions here are one or two key thinkers for all such as St Severus of Antioch and St Jacob of Serugh, who formed the Post-Chalcedonian Basis of Christology and Theology but Each has its own thinkers:
Due to the dependency of a Abune being appointed by Alexandria, lacked a strong Theological movement in the middle ages. It was only in the middle ages that they had a sufficient number of Priests to serve the nation and only at the request of the Ethiopian Emperor himself.
Some of the key figures in the Tewahedo Church during the middle ages were Saint Tekle Haymanot and Iyasus Moa who are both amongst the Fathers of Monasticism in the Ethipoian Community and lived by the rule of St Pachomius.
A key Theological writer in the Ethiopian Community is Samuel of Dabra Wagag. As with the others, it is mainly his life which is preserved though in books such as “The Acts of Samuel of Samuel of Dabra Wagag” some of his teachings are preserved.
It was only really when the Franj tried to convert the Ethiopians to the Latin faith that Ethiopian Philosophy and Theological Education became a key aspect, since they needed to defend their faith against the ongoing attempts of Jesuits and others. One key Thinker in the 17th Century whose writings remain is Zera Yacob. His Treatises give an idea of the Progress of Ethiopian Philosophy and Theology after the middle ages.
The Coptic Church had many Golden ages and dry spells in the Middle Ages, all dependent on which Dynasty ruled Egypt and which foreign invader was attacking.
Most Medieval Coptic writing is Apologetic, and aimd at explaining the faith to the islamic community and especialy the Islamic Philosophers and Theologians of the Era. This led to a vast Arabic Christian Golden Age.
Some of the Great thinkers of the time include Ibn Kabar, the 4 Sons of Al-Assal and Yayha Ibn Ali, all of whom wrote vastly explaining Christian life, the Nature of God and the Nature of the Church.
Yahya Ibn Ali is the Earliest of these (10th C) and wrote apologetic texts on the Nature of God, refuting many Greek philosophical stances. He is more commonly associated with the Syriac Church as he was based in Baghdad though his writing was more of an influence on the Coptic Church due to its Arabic language.
His most important Theological works are on the Incarnation and his linguistic commentaries on the Gospels, which are extremely in depth as both studies of Christian Etymology and Theology.
The Al-Assal brothers are probably the most influencial medieval Coptic writers, ecpecially Abul Fada’il ibn Al-Assal, who is my favourite Coptic Medieval Thinker (Since the Canons are my field of study). Abul Fada’il ibn Al-Assal wrote the Nomocanon which in the Ethiopian Tewahedo tradition is called the ‘Fetha Negast’ or ‘Law of the Kings.’ It compiles the Canons of the fathers, Medieval Patriarchal Canons and Conciliar Canons into the text which was used as a guide for the Church until very Recently. His works are still widely used in the Ethiopian Tradition which uses the Fetha Negast as its key text in Canon Law.
Abu l-Barakat Ibn Kabar is a third recommended thinker in the Coptic Church. He wrote extensively on Liturgical and ritual life in the Coptic Church with his text ‘The lamp in the darkness’, a guide for the Diaconate in the Liturgy, still being regarded as an important reference for those studying the Liturgy.
Much of the time there was a lack of initiative in the Church to write much other than Apologetics. The key points for Coptic writings tend to fall at the start of the Arab rule (due to being free from Byzantine persecution), the Ayyubid Sultanate (as Salah-ud-din’s successors were respectful) and the Ottoman ocupation (as the Mamluk persecution had ended.)
The Syriac Theological Schools remained strong Throughout the Middle Ages. Two thinkers that I recommend to any Orthodox Theologian are Jacob Bar-Salibi and Bar-Hebraus, both from Malatia.
Jacob Bar Salibi is one of the most well known Oriental Orthodox Theologians and quotes the fathers extensively in his works. His most famous pieces are his Biblical Commentaries.
He also wrote many anti-Heretical treatises. I would recommend his piece “Against the Melchites” to any Byzantine Christian as it explains the divergence of Tradition between EO and OO during these times and gives a good Orental perspective on the issues.
Bar Hebraeus is one of my favourite Theological Thinkers of the Era (along with Abul Fada’il ibn Al-Assal) was a Theologian and the Catholicos of the Syriac Church. he wrote extensively on Theology as well as Scientific texts. Of all his texts, the most Fascinating is the Chronicon Ecclesiasticum, which is a history of the world.
One of his key texts is the “Lamp of the Sanctuary” which is unbublished but has viewable copy in the British library if requested. Most of his Theological views state that the Chalcedonian and Non-Chalcedonian communities have no major Dogmatic differences, something that has been supported by Antiochian and Syriac Orthodox heirarchs to this day.