After Theodore attained an appropriate age, he was given a blessing to be ordined to the priesthood. With the blessing of Saint Sergius, Saint Theodore built a church in honor of the Nativity of the Most Holy Theotokos and founded a monastery on the banks of the River Moskva, at the place called Simonovo. Soon the monastery began to attract a throng of people. Saint Theodore built a cell five versts from the Moscow Kremlin, and pursued new ascetical labors, and here disciples gathered around him. Saint Sergius, visiting this place, blessed the founding of a monastery, and Metropolitan Alexis blessed the construction of a church in the name of the Dormition of the Most Holy Theotokos at Novoe Simonovo, which also had its foundations laid in 1379. The old Simonov monastery remained the burial place of monks.
Because of his virtuous life and strict asceticism, Saint Theodore became known in Moscow. The Metropolitan Saint Alexis elevated him to the rank of igumen, and Great Prince Demetrius of the Don chose him as his father confessor. Saint Theodore journeyed to Constantinople several times on church matters for the Russian Metropolitan. On his first journey in 1384, Patriarch Nilus made him an archimandrite. The Simonov monastery was put directly under the Patriarch, thus became stavropegial. In 1387, he was consecrated archbishop and occupied the See of Rostov.
Being the igumen, and then the archimandrite of the Simonov monastery, and despite being occupied with churchly matters, Saint Theodore stalwartly guided those in the monastic life and counted many great and famous ascetics among his disciples. Saints Cyril (June 9) and Therapon (May 27), the future founders of two famous White Lake monasteries, were tonsured at the Simonov monastery. Saint Theodore occupied himself with iconography, and he adorned with icons of his own painting both the Simonov monastery, and many Moscow churches.
At Rostov, Archbishop Theodore founded the Nativity of the Virgin monastery.
The blessed death of the saint occurred on November 28, 1394. His relics are in the Rostov Dormition cathedral.
Saint Theodore is also commemorated on May 23.