One God by nature because of God the Father

Why do we have one God by nature, a single God, despite the fact that they are three Persons, three hypostases – the Holy Trinity? The answer is simple: there is “a single God because a single Father,” according to the Greek Fathers of the Church. For “unless the Father is, neither the Son nor the Spirit is” (Lossky, V. 1976. The mystical theology of the Eastern Church).