The Trinity alone has eternal essence and eternal existence. In the beginning of creation God brought time and space into existence, as is evident from the very term "the beginning." Everything else was made by God in time and space and is comprised of created matter/energy. Created matter is that which was brought forth by God out of non-existence into being in space and time. This includes angels, who although are rightly called incorporeal with regard to the dense matter of the body, are still of created matter (albeit spiritual rather than dense matter), "for in reality only the Deity is immaterial and incorporeal" (St. John of Damascus, On the Orthodox Faith 2.3). This is because we have two definitions of matter, the one definition used to distinguish dense material from that which is created but less dense, or spiritual (also we might say in "energy" form, as St. John points out from Scripture). However, even created spirit has "form and limitation of essence" that is known only to God and is made of created matter-energy. It is here that we find the other more particular definition of matter, distinguishing between God who alone has limitless essence and creation which has bounded essence, and in this case only God is immaterial and all else "we find to be dense and material" (ibid.).
This is highly significant in that it squares completely with science. Although some modern forms of Christianity try to present a "dualistic" form of created existence (i.e. an ontological difference between created matter and spirit), in terms of original Christian ontology, Christian Orthodoxy from ancient times has upheld the monistic form of created existence that we also find in science, distinguishing relatively between dense and spiritual matter, but ontologically upholding it all to be created matter nonetheless. While by the supernatural energy of the Creator all matter was created, nevertheless, by natural forces, matter cannot be created or destroyed, but only its form changed (as we see in Einstein's famous formula E=mc^2).
Among rational created beings we have angels and humans. "Also all that is rational is endowed with free-will. As it is, then, rational and intelligent, it is endowed with free-will: and as it is created, it is changeable, having power either to abide or progress in goodness, or to turn towards evil" (St. John of Damascus On the Orthodox Faith 2.3). St. John, however, notes one difference between humans and angels, that while both may choose which path to trod, only the humans are predisposed to repentance due to the humility of the body. Thus, although not a matter of comparitive dogma, we may deduce that it is easier for an angel not to sin, and yet and it is easier for a human to turn from His wickedness by repentance and live, thereby striking a balance and also explaining what is observed in salvation history.
Creation, Faith, Science and Scripture
Medicine and Science
Wisdom of Sirach (Ecclesiasticus) 38: