The reference point for God’s existence is His Fatherhood not His essence – 2

For in worshiping ‘God of God’ we both confess the distinction of Persons and abide by the Monarchy (Basil the Great, On the Holy Spirit, 18; PG 32).

The source of power is the Father; the power is the Son; the spirit of power is the Holy Spirit (Gregory of Nazianzus, Oratione 23, 11; PG 35).

“In Greek patristic literature, one often finds the idea of causality applied to the person of the Father. The Father is called the cause of the hypostases of the Son and the Holy Spirit, or even the ‘Godhead-source.’ Sometimes He is designated simply as ‘God’ . . . the Father is the personal principle of unity of the Three, the source of their common possession of the same content, of the same essence. The expressions ‘Godhead-source’ and ‘source of the Godhead’ do not mean that the divine essence is subject to the person of the Father, but only that the person of the Father is the basis of common possession of the same essence, because the person of the Father, not being the sole person of the Godhead, is not to be identified with the essence . . . [Thus] the Father is the cause of the other hypostases in that He is not His essence, i.e., in that He does not have His essence for Himself alone . . . the Father, being not merely an essence but a person, is by that very fact the cause of the other consubstantial Persons, who have the same essence as His has . . . The monarchy of the Father thus sets up irreversible relationships, which enable us to distinguish the two other hypostases from the Father, and yet to relate them to the Father, as a concrete principle of unity in the Trinity” (Lossky, V. 1974. In the image and likeness of God, pp. 82-83).

“The Greek Fathers always maintained that the principle of unity in the Trinity is the Person of the Father. As principle of the other two Persons, the Father is at the same time the Source of the relations whence the hypostases receive Their distinctive characteristics. In causing the Persons to proceed, He lays down Their relations of origin . . . in regard to the unique principle of the Godhead” (Lossky, V. 1976. The mystical theology of the Eastern Church, p. 58).