[citation needed] Certain details about his early life are unknown but history records that Isaac had been a hermit living in a small hut in the wilderness outside of Constantinople.

In the year 378, when he heard that the Roman emperor Valens had fallen into the heresy of Arianism and was persecuting the Nicene Christians, deposing bishops, closing some churches, and turning others over to the Arians, Isaac went into the imperial city to confront the emperor.

After several attempts to dissuade the emperor from his persecutions, Isaac prophesied[citation needed] that Valens would "die in flames" because of his actions.

Soon after, on August 9, 378, Valens was defeated at the Battle of Adrianople and died in a fire after taking refuge in a barn.

Isaac is also known as a zealous defender of Christian orthodoxy at the Second Ecumenical Council, convened in Constantinople in 381.