John is in the world, the nephew and disciple of St. Sergius of Radonezh. At the age of 12, he was given to the Radonezh abbot for upbringing and at the same time he was tonsured a monk with the name Theodore. Under the guidance of the saint, St. Theodore grew rapidly in spiritual life and there, at the Trinity Monastery, he took priestly orders. When the young ascetic revealed to his spiritual father his desire to found a special cenobitic monastery, St. Sergius, seeing in his disciple a great ascetic, blessed him to choose a place for a new monastery.
St. Theodore chose the place Simonovo on the banks of the Moskva River, and St. Sergius himself came to see and bless the chosen place. Here, St. Theodore built a church in honor of the Nativity of the Virgin, and in 1380, the heroes of the Battle of Kulikovo, the schema monks Alexander and Andrew, were buried there. The new monastery became known as Simonov Monastery. As abbot of the Simonov monastery, the Monk Theodore, although distracted by church-wide affairs, nevertheless relentlessly led monastic life and educated many ascetics, of whom the most famous are the Monk Kirill Belozersky and the Monk Ferapont Luzhetsky (Mozhaisky).
The saint's deep mind and monastic exploits earned him universal love, and the holy Prince Dimitri Ivanovich Donskoy chose him as his confessor and often entrusted him with important church and state affairs. Thus, in 1381, the saint traveled to Kiev to invite St. Cyprian to the Moscow see; in 1383, he was in Constantinople on an important ecclesiastical mission. In 1388, the Monk Theodore traveled to Constantinople again on business for the Russian Church and returned to Russia in the rank of Archbishop of Rostov. During his brief administration of the Rostov diocese, he founded the Rozhdestvensky Monastery in Rostov and managed it until his death on November 28, 1394.
The saint was a good iconographer and painted many icons during his life, including the image of St. Sergius of Radonezh. The holy relics of St. Theodore rest in the Assumption Cathedral of the Rostov Kremlin.