Today's feast is that of the latter Gregory, who was the bishop of Constantinople, a role which made him a kind of "pope" of the Eastern portion of Christianity. There was much he accomplished during his life, mainly in suppressing the first true heresy of developing Christianity, and in composing the doctrine of the Trinity, a notion still much prized [or derided] for its inclusion and complexity. His greatest literary effort, the Five Theological Orations, was still mandatory reading in seminaries in my day. His legacy is that of an orderly academic discipline in the study of God's mission of salvation.
More of Gregory may be read here and here.
Almighty God, you have revealed to your Church your eternal Being of glorious majesty and perfect love as one God in Trinity of Persons: Give us grace that, like your bishop Gregory of Nazianzus, we may continue steadfast in the confession of this faith, and constant in our worship of you, Father, Son, and Holy Spirit; for you live and reign for ever and ever. Amen.