While a hermit in the east, Isaac heard that the Arians, supported by the Emperor Valens, were persecuting Orthodoxy. Leaving his seclusion, he traveled to Constantinople, where he lived in a small hut. He confronted the Emperor, telling him that if he did not cease his persecutions and embrace the true Faith, disaster would befall him. The Emperor ignored his words, and shortly thereafter was killed in a battle with the Goths. The Emperor Theodosius the Great then came to the throne, restoring peace to the Church. Hearing of Isaac and his prophecy, the Emperor sent for Isaac and prostrated before him. Isaac wished to return to the desert, but was persuaded to remain as a monk in Constantinople. He took part in the Second Ecumenical Council, where he shone in zeal for the Faith; the Third Ecumenical Council made him archimandrite over all the monasteries in the City. (Some say that the monastery founded by him is called the Dalmatian Monastery because it was built by Dalmatus, a wealthy nobleman of the City; others say that it was founded by St Isaac himself and later took its name from Abbot Dalmatus, who succeeded Isaac). In his own lifetime St Isaac was known far and wide as a wonderworker and one endowed with the gift of prophecy.
Saint Isaac is also commemorated in August 3, along with Dalmatus and his son Faustus.