Metropolitan John Zizoulas on the Monarchy of the Father in the Holy Trinity
Among the Greek fathers the unity of God, the one God, and the ontological “principle” or “cause” of the being and life of God does not consist in the one substance of God but in the hypostasis, that is, the person of the Father. The one God is not the one substance but the Father, who is the “cause” both of the generation of the Son and of the procession of the Spirit. Consequently, the ontological “principle” of God is traced back, once again, to the person… The Father out of love-that is, freely-begets the Son and brings forth the Spirit. If God exists, He exists because the Father exists, that is He who out of love freely begets the Son and brings forth the Spirit. Thus God as person-as the hypostasis of the Father-makes the one divine substance to be that which it is: the one God.
* Taken from Being As Communion, pp. 40-41.