Pope John I
reigned gloriously 523–526
May 27 commemorates Pope Saint John I - the first pope to change his name (his birth name was the name of the pagan god Mercury).
Saint John I is notable in that he visited Constantinople as Pope as was immediately greeted by the Emperor and Patriarch as their superior. In fact, the Byzantine Emperor asked to for a second coronation from the hands of the Pope. Moreover, the Patriarch of Constantinople allowed the holy Pope to celebrate a pontifical Latin Mass in the Hagia Sophia - the cathedral church of Constantinople. The date was Easter Day, 19 April, 526.
An interesting fact in the history of East/West relations.