Vladimir Lossky, in his book The Mystical Theology of the Eastern Church argues that Orthodoxy makes three kinds of distinctions in God
1. The three hypostasis
2. The nature or essence (ousia)
3. The energies.
The Son and the Holy Spirit are personal processions, whereas the energies natural processions. The energies are inseparable from the nature, and the nature is inseparable from the three Persons.
These distinctions are of great importance for the Eastern Church’s conception of mystical life for 3 reasons:
First, the distinction of essence and energies 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 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.
Second, the 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 14:23). The communication of God to us in his energies is precisely the meaning of grace (which is a very different hermeneutical grid than the West’s tendency of viewing grace mostly in penal categories). It is by God’s grace, his deifying operations in us, his energies, that he communicates himself to us. 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 (different mode) but nonetheless truly from that in which it is present in its nature.
Third, the distinction between the essences and the energies preserves St. Peter’s words “partakers of the divine nature.” We do not become partakers of the divine nature by becoming another Person in the Trinity (which would be a hypostatic union) nor by our human essence merging into the infinite essence of God (a substantial union). Rather we are united to God in His energies, or by grace.
Lossky also points out that these distinctions do not contradict the apophatic attitude (i.e., the human necessity to describe the indescribable nature of God more in terms of what he is not than what he is) 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.” The distinction between the essence and the energies is due to the antinomy between the unknowable and the knowable, the incommunicable and the communicable. This reveals to us the mystery of God, “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.”