Our starting-point must be the fact that God cannot be named... no mind has yet contained or language embraced God's substance in its fullness. No, we use facts connected with Him to outline qualities that correspond with Him, collecting a faint and feeble mental image from various quarters. Our noblest theologian is not one who has discovered the whole - our earthly shackles do not permit us the whole - but one whose mental image is by comparison fuller, who has gathered in his mind a richer picture, outline, or whatever we call it, of the truth.
Gregory of NazianzusAs 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 Nazianzus