““What is not assumed, is not redeemed”“
Gregory made the statement during the great fourth century debate on the nature of Jesus Christ in an effort to stop the budding Apollinarism heresy.
Apollinarism came about as a way of explaining the Council of Nicene’s (325 AD) decision that Jesus was of “one essence with the Father” In effect, this heresy stated that Jesus had a human body and soul, but a divine mind.
As a response, Gregory coined the phrase “what is not assumed is not redeemed” as a way of saying that Jesus had to have been fully human in order for Him to redeem humanity.
If Jesus was somehow less then fully human, then the parts that were not “assumed” were not “redeemed” – whether those parts be the mind, soul, body or spirit.
The result of this debate was the Nicene–Constantinopolitan Creed of 381:
We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible;
And in one Lord Jesus Christ, the Son of God, the Only-begotten, Begotten of the Father before all worlds, Light of Light, Very God of Very God, Begotten, not made; of one essence with the Father, by whom all things were made:
Who for us men and for our salvation came down from heaven, and was incarnate of the Holy Spirit and the Virgin Mary, and was made man;
And was crucified also for us under Pontius Pilate, and suffered and was buried; And the third day He rose again, according to the Scriptures; And ascended into heaven, and sitteth at the right hand of the Father;
And He shall come again with glory to judge the living and the dead, Whose kingdom shall have no end.
And we believe in the Holy Spirit, the Lord, and Giver of Life, Who proceeds from the Father, Who with the Father and the Son together is worshipped and glorified, Who spoke by the Prophets;
And we believe in one, holy, catholic, and apostolic Church.
We acknowledge one Baptism for the remission of sins.
We look for the Resurrection of the dead, And the Life of the world to come. Amen.