Archbishop of Constantinople, one of the Three Holy Hierarchs.
St. Gregory the Theologian was born in Cappadocia in the fourth century and studied in Athens with his close friend St. Basil the Great. A man of deep prayer and unmatched eloquence, he was called to Constantinople when the city was dominated by Arianism, and through his celebrated theological orations restored the Orthodox faith in the Holy Trinity. He briefly served as Archbishop of Constantinople before withdrawing to a life of quiet and asceticism.
The Church has given the title Theologian only to him, to St. John the Evangelist, and to St. Symeon the New Theologian. One of the Three Holy Hierarchs, he is commemorated on January 25.
Feast day
On the New Calendar, Gregory the Theologian is commemorated on January 25. Churches on the Old (Julian) Calendar keep the feast on February 7 of the civil calendar.
Who celebrates a name day
People with these names keep their name day on this feast:
Grigorios, Grigoris, Gregory, Greg, Grigoria
Also commemorated on January 25
Publius the Ascetic
Venerable, Πούπλιος
Venerable Publius of Syria, founder of a monastery near Zeugma.

Vladimir of Kiev
Hierarch, Βλαδίμηρος Κιέβου
Hieromartyr Vladimir, Metropolitan of Kiev, first of the Russian New Martyrs.
Icon: public domain image from Wikimedia Commons.