Early Life and Monastic Calling
Gennadius was born Gregory in Mogilev to a wealthy family and showed an early devotion to the monastic life, a leaning that troubled his parents. As a young man he departed his home in humble clothing and traveled to Moscow in search of spiritual direction.
After visiting the holy sites of Moscow, Gregory continued on to Novgorod, where he encountered Saint Alexander of Svir. Following Alexander's guidance, he went on to study under Saint Cornelius of Komel, who tonsured him a monk and gave him the name Gennadius.
Foundation of the Kostroma Monastery
Around 1529, Gennadius and Cornelius established the Transfiguration monastery near Lake Sura in the Kostroma forest. As igumen, Gennadius shared in the manual labor of the community, taking part in wood chopping, candle-making, and icon painting.
He practiced severe asceticism, including the wearing of heavy chains. His disciple Iguman Alexis recorded his life between 1584 and 1587, preserving also his spiritual testament, in which Gennadius counseled the monks to keep their Rule, labor diligently, live peacefully, and preserve the monastery's books, exhorting them to 'Strive towards the light, and shun the darkness.'
Gifts and Prophecy
His spiritual dedication was associated with gifts of clairvoyance and miraculous power. A notable prophecy is recorded from a visit Gennadius made to the home of the nobleman Roman Zakharin in Moscow, where he foretold that Zakharin's daughter Anastasia would become Tsaritsa.
The prediction was fulfilled when Ivan the Terrible married Anastasia.
Veneration
Gennadius reposed on January 23, 1565, the day on which he is principally commemorated. He was canonized on August 19, 1646, an event marked by the uncovering of his relics roughly eighty-one years after his repose; that uncovering is itself commemorated on August 19.
He is also venerated in the Yaroslavl region under the alternate toponym Liubimograd (Lyubimgrad), and is numbered among the Synaxis of All Saints of Rostov and Yaroslavl and among the Poshekhonsk Wonderworkers, commemorated on May 23.