St Paul says… ‘I myself with the mind serve the law of God, but with the flesh the law of sin. There is therefore now no condemnation to them that are in Christ Jesus. For the law of the spirit of life in Christ Jesus made me free from the law of sin.’
[...] This God promised by Ezekiel, saying: ‘A new heart also will I give you, and a new spirit will I put within you; and I will take away the stony heart out of your flesh, and I will give you a heart of flesh, and I will put my Spirit within you.’
When has this been fulfilled, save when the Lord came and renewed all things by grace? … Our spirit is renewed; but the Holy Spirit is not simply spirit, but God says it is his Spirit, whereby ours is renewed.
As the Psalmist says in Psalm 103: ‘Thou shalt take away their spirit, and they shall die and return to their dust. Thou shalt put forth thy Spirit, and they shall be created, and thou shalt renew the face of the earth.’
But if it is by the Spirit of God that we are renewed, then the spirit here said to be created is not the Holy Spirit but our spirit…; the Holy Spirit is not created, but…it is our spirit which is renewed in him.
Of this spirit David also prayed in the Psalm: ‘Create in me a clean heart, O God, and renew a right spirit within me.’ Here he is said to create it, but aforetime, as Zechariah says, he formed it: ‘Stretching forth the heavens and laying the foundations of the earth, and forming the spirit of man within him.’
For when that which he formed aforetime had fallen he remade it, coming himself in the creature, when the Word became flesh; so that, in the words of the Apostle, ‘He might create in himself of the twain one new man, who after God had been created in righteousness and holiness of truth’.
For it was not as if another man had been created, other than he who from the beginning was made in God’s image.
But he was counselling them to receive the mind that was remade and renewed in Christ; as is once more made clear through Ezekiel, when he says: ‘Make yourselves a new heart and a new spirit. For why will ye die, O house of Israel? For I have no pleasure in the death of him that dieth, saith the Lord God.’
Athanasius of Alexandria (c.293-373): Letters to Serapion, 1.8-9.