A confusing and somewhat contentious topic central to Eastern Christian thought is the idea of God's energy and essence and how they interact with man. In Catholic-Orthodox discussions there is often a St. Thomas Aquinas vs. St. Gregory Palamas debate, but Catholic theologians of recent times are much more willing to accept the tenets of hesychasm than in previous centuries. In Evangelical circles the very concept calls the Christianity of the Orthodox into question (see this book). Below is an excerpt taken from The Burning Bush from the book The Mystical Theology of the Eastern Church. The word 'dense' came up three times in reference to this book - I caution that one might have to reread the material a few times if the concept is completely foreign.
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.