Saint Theodore was called “Sanctified” because he was the first in his monastery ordained to the priesthood.
Saint Theodore came from Egypt and was the son of rich and illustrious Christian parents. The yearning for monastic life appeared early in him. Once there was a large party at the house of his parents during the feast of Theophany. The boy did not want to take part in the festivities, grieving that because of earthly joys he might be deprived of joys in the life to come. He secretly left home when he was fourteen and entered one of the monasteries.
Hearing about Pachomius the Great [May 15], he burned with the desire to see the ascetic. Saint Pachomius received the young man with love, having been informed by God beforehand about his coming. Remaining at the monastery, Saint Theodore quickly succeeded in all his monastic tasks, particularly in the full obedience to his guide, and in his compassion towards the other brethren. Theodore’s mother, learning that he was at the Tabennisi monastery, came to Saint Pachomius with a letter from the bishop, asking to see her son. Saint Theodore did not wish to break his vow to renounce the world, so he refused to meet with his mother.
Seeing Saint Theodore’s strength of mind and ability, Saint Pachomius once told him to instruct the brethren on Holy Scripture. Saint Theodore was then only twenty years old. He obeyed and began to speak, but some of the older brethren took offense that a new monk should teach them, and they departed.
St. Pachomius the Great & the Angel (from here)
Saint Pachomius said to them, “You have given in to the devil and because of your conceit, your efforts will come to naught. You have not rejected Theodore, but rather the Word of God, and have deprived yourselves of the Holy Spirit.”
Saint Pachomius appointed Saint Theodore as overseer of the Tabennisi monastery, and withdrew to a more solitary monastery. Saint Theodore with filial love continued to concern himself over his instructor, and he looked after Saint Pachomius in his final illness, and when the great abba reposed in the Lord, he closed his eyes. After the death of Saint Pachomius, Saint Theodore directed the Tabennisi monastery, and later on he was at the head of all the Thebaid monasteries. Saint Theodore the Sanctified was famed for his holiness of life and a great gift of wonderworking, and he was well known to Saint Athanasius, Patriarch of Alexandria. Saint Theodore reposed in his old age in the year 368.