For, in the view of apostles and prophets and teachers, the mystery of the divine incarnation has been distinguished as having two points of contemplation natural to it, distinct in all things, inasmuch as on the one hand it is the subsistence of perfect deity, and on the other is demonstrative of full humanity. As long, therefore, as the Word is acknowledged to be in substance one, of one energy, there shall never in any way be known a movement in the two. For while God, who is essentially ever-existent, became by His infinite power, according to His will, sinless man, He is what He was, in all wherein God is known; and what He became, He is in all wherein man is known and can be recognised. In both aspects of Himself He never falls out of Himself, in His divine activities and in His human alike, preserving in both relations His own essentially unchangeable perfection.
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 5 (Roberts and Donaldson, 1886), public domain (Public domain).