During his papal reign Italy was ruled by the Ostrogothic King Theodoric. Like many of his fellow tribesmen, the king adhered to the Arian heresy, holding that Christ was a created being rather than the Second Person of the Holy Trinity.
Arianism had originated in the Eastern half of the Roman Empire during the fourth century, and subsequently spread among the Western Goths. By the sixth century the heresy was weak in the East, but not dead.
In 523, the Byzantine Emperor Justin I ordered Arian clergy to surrender their churches into orthodox Catholic hands. In the West, meanwhile, Theodoric was angered by the emperor's move, and responded by trying to use the Pope's authority for his own ends.
Pope John was thus placed in an extremely awkward position. Despite the Pope's own solid orthodoxy, the Arian king seems to have expected him to intercede with the Eastern emperor on behalf of the heretics. John's refusal to satisfy King Theodoric would eventually lead to his martyrdom.
John did travel to Constantinople, where he was honored as St. Peter's successor by the people, the Byzantine Emperor, and the Church's legitimate Eastern patriarchs. (The Church of Alexandria had already separated by this point.) The Pope crowned the emperor, and celebrated the Easter liturgy at the Hagia Sophia Church in April of 526.
But while John could urge Justin to treat the Arians somewhat more mercifully, he could not make the kind of demands on their behalf that Theodoric expected.