“. . . it is not possible to behold the person of the Father otherwise than by fixing the sight upon it through his Image; and the image of the person of the Father is the Only-begotten, and to Him again no man can draw near whose mind has not been illuminated by the Holy Spirit, what else is shown from this but that the Holy Spirit is not separated from any operation which is wrought by the Father and the Son? Thus the identity of operation in Father, Son, and Holy Spirit shows plainly the undistinguishable character of their substance. So that even the name of the Godhead does indicate nature, the community of substance shows that this appellation is properly applied also to the Holy Spirit.”
From his “On The Holy Trinity”, vol. 5 “Nicene and Post-Nicene Fathers” ed. Phillip Schaff, D.D., LL.D., and Henry Wace, D.D.