Eastern Orthodoxy believes in one triune God. This doesn’t mean we believe in three separate Gods but that there are three Persons who comprise one God. Orthodox teaching of the trinity is also a bit distinct from how most Western Christians perceive it. Here’s how:

1) Essence and Energies

In Orthodoxy, God is distinguished into into the different parts: His three hypostases, His essence, and His energies. The Son and the Holy Spirit are “personal processions” while the energies are “natural processions.” However all three, the hypostases, essence/nature, and energies, are inseparable from each other. According to the notable 20th century Orthodox theologian, “these distinctions are of great importance for the Eastern Church’s conception of mystical life.” He lists out the following

“1. The doctrine of the energies, ineffably distinct from the essence, is the dogmatic basis of the real character of all mystical experience. God, who is inaccessible in His essence, is present in His energies ‘as in a mirror,’ remaining invisible in that which He is; ‘in the same way we are able to see our faces, themselves invisible to us in a glass,’ according to a saying of St. Gregory Palamas. (Sermon on the Presentation of the Holy Virgin in the Temple). Wholly unknowable in His essence, God wholly reveals Himself in His energies, which yet in no way divide His nature into two parts–knowable and unknowable–but signify two different modes of the divine existence, in the essence and outside of the essence.

2. This doctrine makes it possible to understand how the Trinity can remain incommunicable in essence and at the same time come and dwell within us, according to the promise of Christ (John xiv, 23). The presence is not a causal one, such as the divine omnipresence in creation; no more is it a presence according to the very essence–which is by definition incommunicable; it is a mode according to which the Trinity dwells in us by means of that in itself which is communicable–that is to say, by the energies which are common to the three hypostases, or, in other words, by grace–for it is by this name that we know the deifying energies which the Holy Spirit communicates to us. He who has the Spirit, who confers the gift, has at the same time the Son, through whom every gift is transmitted to us; he also has the Father, from whom comes every perfect gift. In receiving the gift–the deifying energies–one receives at the same time the indwelling of the Holy Trinity–inseparable from its natural energies and present in them in a different manner but none the less truly from that in which it is present in its nature.

3. The distinction between the essences and the energies, which is fundamental for the Orthodox doctrine of grace, makes it possible to preserve the real meaning of St. Peter’s words ‘partakers of the divine nature.’ The union to which we are called is neither hypostatic–as in the case of the human nature of Christ–nor substantial, as in that of the three divine Persons: it is union with God in His energies, or union by grace making us participate in the divine nature, without our essence becoming thereby the essence of God. In deification we are by grace (that is to say, in the divine energies) all that God is by nature, save only identity of nature . . ., according to the teaching of St. Maximus (De ambiguis). We remain creatures while becoming God by grace, as Christ remained God in becoming man by the Incarnation.

These distinctions in God which are made by the theology of the Eastern Church do not in any way contradict its apophatic attitude in regard to revealed truth. On the contrary, these antinomical distinctions are dictated by a concern for safeguarding the mystery, while yet expressing the data of revelation in dogma. Thus, as we have seen in the doctrine of the Trinity, the distinction between the persons and the nature revealed a tendency to represent God as a ‘monad and triad in one’, with the consequence that the domination of the unity of the nature over the trinity of the hypostases was avoided, as was the elimination or minimizing of the primordial mystery of the identity-diversity. In the same way, the distinction between the essence and the energies is due to the antinomy between the unknowable and the knowable, the incommunicable and the communicable, with which both religious thought and the experience of divine things are ultimately faced. These real distinctions introduce no ‘composition’ into the divine being; they signify the mystery of God, who is absolutely one according to His nature, absolutely three according to His persons, sovereign and inaccessible Trinity, dwelling in the profusion of glory which is His uncreated light, His eternal Kingdom which all must enter who inherit the deified state of the age to come.” (The Mystical Theology of the Eastern Church; pg. 85-87)

2) One Essence or Two?

Fourth out of the “big seven” ecumenical councils dealt with the christology of Christ, namely the third to sixth. It started with the heretical bishop, Nestorius, claiming that there are two distinct hypostases in the Incarnate Christ, the one Divine and the other human. The teaching of all churches that accept the Third Ecumenical Council of Ephesus is that in the Incarnate Christ is a single hypostasis, God and man at once. That doctrine is known as the Hypostatic union.

Instead Nestorianism believes the human and divine essences of Christ are separate and that there are two persons, the man Jesus Christ and the divine Logos, which dwelt in the man, thereby detaching Christ’s divinity and humanity into two persons existing in one body, thereby denying the reality of the Incarnation.

By the time of the fourth ecumenical a new, but opposite, heresy had arisen. Monophysitism. Its chief proponent was the monk Eutyches, who stated that in the person of Jesus Christ the human nature was absorbed into the divine nature like a cube of sugar dissolves in a cup of water. Therefore, Christ was left with only one essence, the Divine (Greek mono- one, physis – nature/essence).

Eutyches formulated this as a response to Nestorianism. He had thought he was elaborating the points of St Cyril of Alexandria. But this is completely wrong, Cyril never wrote that Christ’s human nature “dissolved” leaving him only with a divine one.

Instead, the Alexandrian state established the christology known as miaphysitism. That in the one person of Jesus Christ, Divinity and Humanity are united (not dissolved) in one “nature” (“physis”), the two being united without separation, without confusion, and without alteration.

In other words, the incarnate Christ has one essence, but that essence is of the two essences, divine and human, and retains all the characteristics of both. This is exactly what the fourth ecuenical council affirmed in condemning the monophysites:

while Christ is a single, undivided person, He is not only from two natures but in two natures. The bishops acclaimed the Tome of St. Leo the Great, Pope of Rome (died 461), in which the distinction between the two natures is clearly stated, although the unity of Christ’s person is also emphasized. In their proclamation of faith they stated their belief in ‘one and the same son, perfect in Godhead and perfect in humanity, truly God and truly human… acknowledged in two natures unconfusedly, unchangeably, indivisibly, inseparably; the difference between the natures is in no way removed because of the union, but rather the peculiar property of each nature is preserved, and both combine in one person and in one hypostasis.

3) A Triune God

From LAcopts.org

“God is one in essence, yet three, Father, Son, and Holy Spirit, in persons. There is eternally in God true unity, combined with genuinely personal differentiation: the term ‘essence’, ‘substance’, ‘being’, or ‘nature’ indicates the unity, and the term person indicates the differentiation.

Father, Son, and Holy Spirit are one in essence, not merely in the sense that all three are examples of the same group or general class (as with human beings), but in the sense that they form a single, unique, specific reality. There are no variety packages of non-essential characteristics. There is in this respect then an important difference between the sense in which the three divine persons are one, and the sense that three human persons may be termed one. Humans, however closely they co-operate, each retain their own will and their own energy. In short, they are three men and not one man. But in the case of the three persons of the Trinity, there is distinction but never separation. Father, Son, and Holy Spirit, have only ONE will and not three, only ONE energy and not three. None of the three ever acts separately, apart from the other two. They are not three Gods, but one God.

Yet, although the three persons never act apart from each other, there is in God genuine diversity as well as specific unity. We believe that this threefold differentiation in God’s outward action reflects a threefold differentiation in His inner life. Father, Son, and Holy Spirit are not just “modes” or “moods” of the Divinity, not just masks God assumes for a time in His dealings with creation and then lays aside. They are on the contrary coequal and coeternal persons.

A human father is older than his child, but when speaking of God as ‘Father’ and ‘Son’ we are not to interpret the terms in this literal sense. We affirm of the Son, “There was never a time when he was not.” And the same is said of the Holy Spirit.”