One God: One Father
First of all, it is the Church’s teaching and its deepest experience that there is only one God because there is only one Father.
In the Bible the term “God” with very few exceptions is used primarily as a name for the Father. Thus, the Son is the “Son of God,” and the Spirit is the “Spirit of God.” The Son is born from the Father, and the Spirit proceeds from the Father—both in the same timeless and eternal action of the Father’s own being.
In this view, the Son and the Spirit are both one with God and in no way separated from Him. Thus, the Divine Unity consists of the Father, with His Son and His Spirit distinct from Himself and yet perfectly united together in Him.
One God: One Divine Nature and Being
What the Father is, the Son and the Spirit are also. This is the Church’s teaching. The Son, born of the Father, and the Spirit, proceeding from Him, share the divine nature with God, being “of one essence” with Him.
Thus, as the Father is “ineffable, inconceivable, invisible, incomprehensible, ever-existing and eternally the same” (Divine Liturgy of St. John Chrysostom), so the Son and the Spirit are exactly the same. Every attribute of divinity which belongs to God the Father—life, love, wisdom, truth, blessedness, holiness, power, purity, joy—belongs equally as well to the Son and the Holy Spirit. The being, nature, essence, existence and life of God the Father, the Son and the Holy Spirit are absolutely and identically one and the same.
One God: One Divine Action and Will
Since the being of the Holy Trinity is one, whatever the Father wills, the Son and the Holy Spirit will also. What the Father does, the Son and the Holy Spirit do also. There is no will and no action of God the Father which is not at the same time the will and action of the Son and the Holy Spirit.
In Himself, in eternity, as well as towards the world in creation, revelation, incarnation, redemption, sanctification, and glorification—the will and action of the Trinity are one: from the divine Father, through the divine Son, in the divine Holy Spirit. Every action of God is the action of the Three. No one person of the Trinity acts independently of or in isolation from the others. The action of each is the action of all; the action of all is the action of each. And the divine action is essentially one.
One God: One Divine Knowledge and Love
Since each person of the Trinity is one with the others, each knows the same Truth and exercises the same Love. The knowledge of each is the knowledge of all, and the Love of each is the Love of all.
If taken in distinction, each person of the Trinity knows and loves the others with such absolute perfection, knowledge, and love that there is nothing unknown and nothing unloved of each in the others, and all in all. Thus, if the creaturely knowledge of men can unite minds in full unanimity, and if the creaturely love of men can bring the divided together into one heart and one soul and even one flesh, how incomparably more perfect and absolutely uniting must be the oneness when the Knowers and Lovers are eternal and divine.