Saint Sergius left no theological writings behind. However, as a priest and abbot of a monastery, he undoubtedly preached, commented on Scripture, and taught his brethren. Evidence of St. Sergius' theological gift, hidden for centuries, is the famous icon of the Holy Trinity by Andrei Rublev, which he undoubtedly inspired. It is a great sign that not only in life, but also in death, Sergius revealed a special connection with the Trisagion hymn.
Repose of Venerable Sergius, Abbot of Radonezh
On October 8, the Russian Orthodox Church celebrates the repose of St. Sergius of Radonezh (1314–1392). According to his biography, six months before his death, when Sergius was 78 years old, he gathered the brethren of the monastery around him and handed over leadership to his disciple Nikon. In September, he fell seriously ill and summoned the brethren to him for the last time. "I am departing to God, who is calling me. I entrust you to the Almighty Lord and His Most Pure Mother. May She be your refuge and a wall against the arrows of the evil one," were his last words. The day of Sergius' death is also marked in church calendars as the day of the great and terrible earthquake in Constantinople. This event is described by St. Dimitri of Rostov in his "Lives of the Saints"; it is also mentioned in the liturgical calendar of the Orthodox Church in America and other sources.
The natural disaster ravaged the Byzantine capital for six months. This was during the long reign of Emperor Theodosius II the Younger (402–450), when Saint Proclus (390–447), revered by the people as a disciple of John Chrysostom (347–407), was bishop of the capital. At the time of John's death, Proclus was about seventeen years old. The saint's biography says that he was an unwitting witness, as if in a mysterious vision, to John's work on preaching the apostolic epistles, with the Apostle Paul himself standing nearby and dictating the words to him. At first, neither Proclus recognized Paul, nor did John know that someone was standing next to him. Circumstances helped to clarify this. This remarkable episode testifies to the respect that Proclus himself enjoyed in his mature years in the perception of his Constantinople flock.
Shortly before his death, in a city ravaged by natural disasters, he organized a great religious procession. During the procession, a young man was lifted up to heaven and saw angelic choirs singing the hymn "Trisagion" to God. The prayer was taken up by the people with the addition: "Have mercy on us." Like the vision of the intercession of the Theotokos several centuries later, which was seen by Saint Andrew, the famous ascetic and fool for Christ, this vision of the Trisagion became the apotheosis of Proclus' long episcopate in Constantinople, which began in very difficult conditions back in 334.
After the Fourth Ecumenical Council in Chalcedon (451), the Trisagion acquired two completely different theological interpretations. Supporters of the Council, primarily the Church of Constantinople, were convinced that this prayer was addressed to the Holy Trinity, while opponents, primarily the Churches of Alexandria and Syria, claimed that the Trisagion was exclusively a Christological hymn. The latter rejected the historicity of the miracle of St. Proclus, while the Constantinopolitans, in response, introduced a liturgical celebration in honor of deliverance from the earthquake and the vision associated with it.
It is generally accepted that it was from this moment that the Trisagion began to be used as the central hymn of the Divine Liturgy before the Eucharist. Although the origin of this text is undoubtedly biblical and early Christian, the birthday of this hymn in the liturgy of the Orthodox Church is September 25, the day when the Russian Church solemnly celebrates the repose of St. Sergius. This means that Sergius was born into eternity on the birthday of the Trisagion.