In his Exact Exposition of the Orthodox Faith (Book IV, Chapter 8), the 8th century Doctor of the Church St. John Damascene speaks of Christ as Firstborn and Only-Begotten as a result of the union of the two Natures in the one Divine Person of the Word. What is important to note is that Only-Begotten refers to the Divinity of Christ for He is truly begotten of the Father from all in eternity as the eternal Son; whereas Firstborn refers to the Sacred Humanity of Christ born in time of the Θεοτοκος – Mother of God. The Saint is explicitly referring to the Pauline passages of Christ being the Firstborn among many brethren (Rm 8:29 – see my comments on this passage here) and the Firstborn of all creation (Col 1:16 – see my comments this passage here).
Here is the passage of St. John Damascene:
He who is first begotten is called first-born(1), whether he is only-begotten or the first of a number of brothers. If then the Son of God was called first-born, but was not called Only-begotten, we could imagine that He was the first-born of creatures, as being a creature(2). But since He is called both first-born and Only-begotten, both senses must be preserved in His case. We say that He is first-born of all creation(3) since both He Himself is of God and creation is of God, but as He Himself is born alone and timelessly of the essence of God the Father, He may with reason be called Only-begotten Son, first-born and not first-created. For the creation was not brought into being out of the essence of the Father, but by His will out of nothing(4). And He is called First-born among many brethren(5), for although being Only-begotten, He was also born of a mother. Since, indeed, He participated just as we ourselves do in blood and flesh and became man, while we too through Him became sons of God, being adopted through the baptism, He Who is by nature Son of God became first-born amongst us who were made by adoption and grace sons of God, and stand to Him in the relation of brothers. Wherefore He said, I ascend unto My Father and your Father(6). He did not say “our Father,” but “My Father,” clearly in the sense of Father by nature, and “your Father,” in the sense of Father by grace. And “My God and your God(7).” He did not say “our God,” but “My God:” and if you distinguish with subtle thought that which is seen from that which is thought, also “your God,” as Maker and Lord.
Obviously Christ is not the “First” born chronologically in His Humanity. This indicates that He was the Firstborn of all creation in the divine intention – God, outside of time, willed the Incarnation first and all else is ordered to the Word-Incarnate. Calling Christ the Firstborn makes no sense apart from the absolute primacy of Christ’s predestination.
Let me conclude by posting the brief video of how I explained this about 12 years ago (when I was a lot younger and had a lot less gray hair)…