What is the meaning of the words “of one substance (essence) with the Father?”
The real and only issue behind the use of “essence” (ousia) in theology was to safeguard the absolute difference between created and uncreated. To confess the Son homoousios is to confess that he stands, with the Father, entirely on the Creator’s side of that divide.
The word homoousios was adopted at Nicaea to declare that the Son is co-eternal and co-uncreated with God the Father. As a modern Orthodox scholar observes, St Athanasius and the Cappadocians wrote of the Trinity under the immediate challenge of Arianism, “concerned about defending the uncreated nature of the Son and the Spirit deriving from the very being of God, as they see these truths affirmed by the witness of the Bible and the worship of the Church” (Theodore Stylianopoulos, The Greek Orthodox Theological Review, 31 [1986], pp. 255–288). Doctrine here simply guards doxology: the Church defined what she had always prayed.
There is only one true God, who is eternally, consubstantially, and without division with his Only-Begotten Son and his Holy Spirit: “this is eternal life: that they should know you, the only true God, and him whom you have sent” (Joh 17:3; see Q003, Q083); “we are in him who is true, in his Son Jesus Christ. This is the true God, and eternal life” (1Jo 5:20; see Q093, Q284).
