The Father is Father and not Son.
The Son is Son and not Father.
The Holy Spirit is Spirit and not Father or Son.
For the individuality is unchangeable. How, indeed, could individuality continue to exist at all if it were ever changing and altering?
Wherefore the Son of God became Son of Man in order that His individuality might endure.
For since He was the Son of God, He became Son of Man, being made flesh of the holy Virgin and not losing the individuality of Sonship.
Further, the Son of God became man in order that He might again bestow on man that favour for the sake of which He created him.
For He created him after His own image, endowed with intellect and free-will, and after His own likeness, that is to say, perfect in all virtue so far as it is possible for man’s nature to attain perfection.
For the following properties are, so to speak, marks of the divine nature: viz. absence of care and distraction and guile, goodness, wisdom, justice, freedom from all vice.
He placed man in communion with Himself – for having made him for incorruption (Wisd. 2:23), He led him up through communion with Himself to incorruption.
Through the transgression of the command we confused and obliterated the marks of the divine image, and, having become evil, we were stripped of our communion with God – for what communion hath light with darkness (2 Cor. 6:14)?
And, having been shut out from life we became subject to the corruption of death.
After all this, since He gave us to share in the better part, and we did not keep it secure, He shares in the inferior part, I mean our own nature.
He does this in order that – through Himself and in Himself – He might renew that which was made after His image and likeness;
and that He might teach us, too, the conduct of a virtuous life, making through Himself the way thither easy for us;
and that He might by the communication of life deliver us from corruption, becoming Himself the firstfruits of our resurrection;
that He might renovate the useless and worn vessel calling us to the knowledge of God;
and that He might redeem us from the tyranny of the devil, and might strengthen and teach us how to overthrow the tyrant through patience and humility.
John Damascene (c.675-749): De Fide Orthodoxa 4,4 [slightly adapted].