“We beheld His glory, not of the Son as such, but as of the only-begotten Son of the Father. He, had all the title of that excellency and value in everything. All that that was to the Father was with Him. It is His personal glory made visible in flesh.”
“The difference between μονογενής (only begotten), and πρωτότοκος (first-born), is that the first is His relationship to God eternally; the second is His relationship to other things. Thus ‘I will make him my first-born, higher than the kings of the earth.’ (Psalm 89) This is not what He is essentially.” (Notes on John, Coll. Writings, J. N. D., vol. 25. p. 334.)
A comparison of the texts that speak of the Lord as the first-born, and as the only-begotten Son of the Father, will abundantly confirm the above.
We only find the highest title of the Son in John, when the church had failed as a testimony on earth; then the Spirit by John, brings out the glory of the eternal Son, as the only-begotten of the Father,
“And the Word was made flesh, and: dwelt among us, (and we beheld his glory, the glory as of the only-begotten of the Father), full of grace truth.” (John 1:1414And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (John 1:14)) “No man hath seen God, any time; the only-begotten Son, which is in the bosom of the Father, He hath declared him.” (Ver. 18,) “God so loved the world that he gave his only-begotten Son.” (John 3:1616For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (John 3:16).) “He that believeth not is condemned already, because he Hath not believed in the name of the, only begotten Son of God.” (Ver. 18.) “In this was manifested the Love of God toward us, because that”, ‘God sent his only-begotten Son into the world that we might live through Him.’ (1 John 4,.9.) We can only worship and adore—He was in this ineffable-relation to the Father the beginning. Such was the eternal Son the Father gave to die for our sins. Amazing grace!
The word first-born, on the other hand, is used by the same Spirit when speaking of his preeminence since, and over creation, in reference to persons or things. “Who is the image of the invisible God, the first-born of every creature. For by him were all things created,” “And he is the head of the body, the church; who is the beginning, the first-born from the dead, that in all. things he might have the pre-eminence.” (Col. 1:15-1815Who is the image of the invisible God, the firstborn of every creature: 16For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him: 17And he is before all things, and by him all things consist. 18And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence. (Colossians 1:15‑18).)
So Psalm 89:2727Also I will make him my firstborn, higher than the kings of the earth. (Psalm 89:27) as above, “Also I will make him my first-born, higher than the kings of the earth.” This does not speak of His eternal relationship, but His position on or over the earth. And again, “That he might be the first-born among many brethren.” Thus the scripture is perfectly uniform in the use of these two distinctive aspects of His glory. It would be a sad mistake to confound them.
There remains another scripture (Psalm 2:77I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee. (Psalm 2:7)) which foretells the incarnation of the Son, “Thou art my Son, this day have I begotten thee.” The term only-begotten is not used here. Neither in the quotation of this in Heb. 1:55For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son? (Hebrews 1:5), but “Thou art my Son, this day have I begotten thee.” “And again when he bringeth in the first begotten into the world,” &c. How careful the Holy Ghost has been to keep the varied glories of the Son distinct. Whether His eternal relationship as the only-begotten of the Father—the only-begotten Son in the bosom of the Father, and as such sent that we might have eternal life; or as having the first-born pre-eminence over all creation, or as the incarnate Son. May our hearts receive each of those relationships and glories with adoring gratitude and worship.