Founder and abbot of Sakkoudion
Plato established the monastery of Sakkoudion near Bithynian Mount Olympus around 782-783 and governed it as its first abbot. Under his guidance the community grew, and its church was dedicated to Saint John the Theologian.
He was known for a strict ascetic life. Later accounts describe practices of severe self-denial, including the wearing of an iron chain and a diet of minimal food, in keeping with the rigorous monastic ideal he pursued and taught.
The Moechian controversy
Plato is especially remembered for his part in the dispute known as the Moechian controversy, which arose from Emperor Constantine VI's divorce of his first wife and remarriage to Theodote in 795. Plato and Theodore the Studite held that the divorce was unlawful and the new marriage adulterous, and they refused communion with those who sanctioned it, including the priest who performed the ceremony.
For this resistance to imperial pressure on the canons of the Church, Plato was imprisoned. The controversy made him, with Theodore, a leading voice for the strict application of the Church's discipline against accommodation of the emperor, and it is the principal reason he is honored as a confessor.
Spiritual father of Theodore the Studite
Plato was the uncle and primary spiritual father of Saint Theodore the Studite. After the Seventh Ecumenical Council he summoned Theodore and Theodore's brothers to the monastic life and received them at Sakkoudion, where Theodore confessed to him daily and followed his counsel.
When Plato withdrew from the active leadership of the monastery, Theodore succeeded him as abbot, and in his final years Plato lived under Theodore at the Studion in Constantinople. Theodore honored him with the name of father, reflecting the depth of his formative influence.