As far as we can reach, He Who Is, and God, are the special names of His Essence; and of these especially He Who Is, not only because when He spoke to Moses in the mount, and Moses asked what His Name was, this was what He called Himself, bidding him say to the people 'I Am has sent me' (Ex. 3:14), but also because we find that this Name is the more strictly appropriate.
Gregory of NazianzusMore Gregory of Nazianzus quotes
If anyone does not believe that Holy Mary is the Mother of God, he is severed from the Godhead. If anyone should assert that He passed through the Virgin as through a channel, and was not at once divinely and humanly formed in her (divinely, because without the intervention of a man; humanly, because in accordance with the laws of gestation), he is in like manner godless.
Gregory of Nazianzus