A viewer informed me (and I since found this confirmed): "this is probably Saint Gregory of Nyssa - my favourite Saint and Theologian, brother of Saint Basil. Nowadays his profound works attract a lot of interest."

From the Enc. Brit.: Gregorius Nyssenus philosophical theologian and mystic, leader of the orthodox party in the 4th-century Christian controversies over the doctrine of the Trinity. Primarily a scholar, he wrote many theological, mystical, and monastic works in which he balanced Platonic and Christian traditions. [...] Platonic and Christian inspiration combine in Gregory's ascetic and mystical writings, which have been influential in the devotional traditions of the Eastern Orthodox church and (indirectly) of the Western church. His Life of Macrina blends biography with instruction in the monastic life. On Virginity and other treatises on the ascetic life are crowned by the mystical Life of Moses, which treats the 13th-century-BC journey of the Hebrews from Egypt to Mount Sinai as a pattern of the progress of the soul through the temptations of the world to a vision of God. A notable emphasis of Gregory's teaching is the principle that the spiritual life is not one of static perfection but of constant progress. His greatest achievement is his remarkably balanced synthesis of Hellenic (Greek) and Christian traditions, in an age when both were represented by vigorous and acute minds.

Gregory did not, however, neglect his practical and pastoral duties, as is attested by his preserved letters and sermons. Many of the latter were written in praise of the saints venerated in Cappadocia or to celebrate the great days of the church year. Others, such as Gregory's attacks on usury and on the postponement of Baptism, deal with ethical problems of the church in his time. His more intimate discourses on the Lord's Prayer and the Beatitudes (Matthew 5:3–12) combine ethical and devotional interests, as does his commentary on the Song of Solomon. Gregory disliked attending gatherings of bishops but was periodically invited to preach at such occasions. His last public appearance was at a council at Constantinople. Gregory's ecclesiastical career was less successful than those of Basil and Gregory of Nazianzus, but his work as scholar and writer was creative, and in the 20th century it was rescued from undeserved neglect.