Episcopate and the First Ecumenical Council
Alexander served as bishop of Byzantium from around 314, succeeding Metrophanes. According to the sources, he had become vicar to Metrophanes some years earlier, when already advanced in years. When the city of Byzantium was renamed Constantinople in 330, Alexander became the first bishop to hold the see under its new name.
Though elderly, Alexander represented his predecessor Metrophanes at the Council of Nicaea in 325, since Metrophanes was unable to attend on account of his age and infirmity; Alexander was regarded as his destined successor. At the council he supported Alexander of Alexandria in the defense of the Trinitarian position against the teaching of Arius.