John of Damascus — on Col 1:15 (An Exact Exposition of the Orthodox Faith, Book IV)

“He who is first begotten is called first-born, 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. 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 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. And He is called First-born among many brethren, 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.”