Athanasius of Alexandria

Christ is the true Son, and so when we receive the Spirit, we are made sons. For it says; ‘you did not receive the spirit of slavery leading you back into fear, but you have received the Spirit of adopted sonship’ [Romans 8:15]. But when we are made sons in the Spirit, it is clear that we are called children of God in Christ... And when the Spirit is given to us-the Saviour said: ‘Receive the Holy Spirit’ [John 20:22]- God [The Father] is in us... But when God is in us, the Son is also in us. For the Lord Himself said: ‘I and the Father will come and make our home with him’ [John 14:23]. Next, the Son is life-for He said: ‘I am the life’ [John 14:6]- and so we are said to be given life in the Spirit... But when we are given life in the Spirit, Christ Himself is said to live in us. For it says: ‘I am crucified with Christ. It is no longer I who live, but Christ who lives in me.’ (Galatians 2:19-20). - "Letters to Separion On the Spirit, Letter 1, Chapter 19"

Yet the love of God for humanity is such that by grace He becomes Father of those in relation to whom he had previously only been Maker. He becomes their Father when created beings receive ‘into their hearts the Spirit of the Son crying out, ‘Abba, Father’ [Gal 4.6]., as the apostle says. These are the ones who, by receiving the Word, receive authority from him ‘to become the children of God’ (John 1.12). Being creatures by nature, they would not become ‘sons’ except by receiving the Spirit of the natural and true Son. So it was in order to bring this about and to make humanity receptive of divinity that ‘The Word became flesh’ (John 1.14)… Accordingly, the Father calls ‘sons’ those in whom He sees His own Son and He says, ‘I begot’ since ‘begetting’ signifies ‘sons’ while ‘making’ is indicative of the works. Therefore we are not begotten first but made [created], for it is written, ‘Let us make humanity’ (Gen 1.26). But when we later receive the grace of The Spirit, we are henceforth said to also be begotten. - "Against the Arians, 2.59"