This feast was instituted during the reign of Alexis I Comnenus (1081 1118). A dispute arose in Constantinople among various prominent citizens and clergy, about which of the three Fathers Basil the Great, Gregory the Theologian, or St John Chrysostom was the greatest. In typically Byzantine fashion, the people of the City joined into the dispute, which became more animated, dividing the populace into three hostile factions styling themselves Basilians, Gregorians and Joannites. At last, desiring to restore peace to the City and the Church, the three holy hierarchs themselves appeared to the monk John Mauropus; they revealed to him that they stand together in harmony and in equal glory before the heavenly throne, and instructed him to compose a common service for the three of them. Saint John (Mauropus) obeyed, and chose January 30 as the date of the commemoration, since each of the three hierarchs is commemorated separately in January.

“Saint Peter was a humble, devout and peace-loving man, unlike his father, Tsar Symeon the Warrior (d. 927), during whose reign there had been perpetual warfare. By contrast, Peter’s long reign was peaceful, and notable for the restoration of good relations with Byzantium and with the West. Peter married Maria, the grand-daughter of the Emperor Romanus Lecapenus, who recognized him as basileus (tsar or king), and he obtained independence from Constantinople for the Bulgarian Church with its own Patriarch. He had a great love for Saint John of Rila (19 Oct.), whom he would often consult, and he kept in touch with renowned ascetics of the time like Saint Paul of Latros (15 Dec.). The King acted energetically against the Bogomil heresy, an offshoot of Manicheism, by which some of his people, lacking sufficient instruction in the faith, were being misled. He called a council in order to condemn the heresy and reassert Christian principles. Nevertheless, the infection was to remain active for many years in Bulgaria. Following the invasion of the north of his Kingdom by Prince Svyatoslav of Kiev in 969, Peter abdicated and became a monk. He died in the following year, having consecrated his final days to God alone.� (Synaxarion)

A note on the Bogomils: The Bogomils flourished in the Eastern Europe as an organized church from the 10th to the 15th century. In theology they were dualistic, incorporating some Manichean and Gnostic ideas from the Paulicians. They were nationalistic and gained much support through their opposition to Byzantine dominance over the Slavic peoples. They disappeared as an organized body around the fifteenth century, but elements of their beliefs persisted in popular thinking for many centuries afterward.