Saint Theodosius' greatest achievement has been the introducing of the monastic rule of Saint Theodore the Studite in the Monastery of the Caves from whence it spread to all the monasteries of the Russian Orthodox Church. According to the Primary Chronicle:
"...the monastery was completed during the abbacy of Barlaam...When Barlaam had departed the brethren...visited the aged Athony [founder of the Monastery of the Caves, who was now living in deep seclusion] with the request that he should designate a new abbot for them. He inquired whom they desired. They replied that they required only the one designated by God and by his [Anthony's] own selection. Then he inquired of them: 'Who among you is more obedient, more modest, and more mild than Theodosius? Let him be your abbot.' The brethren rejoiced...and thus they appointed Theodosius to be their abbot.
"When Theodosius took over in the monastery, he began to practice abstinence, fasting, and tearful prayer.... He also interested himself in searching out monastic rules. There was in Kiev at the time a monk from the Studion Monastery named Michael, who had come from Greece.... Theodosius inquired of him the practices of the Studite monks. He obtained their rule from him, copied it out, and established it in his own monastery to govern the chanting of monastic hymns, in making reverences, reading of the lessons, behavior in church, the whole ritual, conduct at the table, proper food for special days, and to regulate all else according to prescription.
"After obtaining all this information, Theodosius thus transmitted it to his monastery, and from the latter all others adopted the same instruction. Whereas the Monastery of the Caves is honored among the oldest of them all.
Theodosius has been glorified (canonized) as a saint by the Russian Orthodox Church His main feast day is May 3, the date of his repose. His relics were discovered by St. Nestor the Chronicler, on August 14, 1091, and were found to be incorrupt. The relics were transferred to the main catholicon (cathedral) of the monastery, and a second annual feast day was established in commemoration of this event.