Do we have any proof from the Bible that the second Person of the Trinity was the Only-begotten Son of the Father before His incarnation?
The classic Bible verse to clearly declare that Jesus was the only-begotten son of God before the incarnation is John 1:1-2, declaring Jesus to be fully God, eternally with his Father.
From this eternity:
Notice the words ‘One and Only’.
So Jesus is the Eternal Word, of the Father and is God of very God. Now the eternal word of God is the express image ‘coming out’ of the Father which is his One and only Eternal Son.
God the Father Himself declared this Jesus to be his eternal Son, during the Baptism of John:
Now the only-begotten Son is not a fictitious rapture of his human nature in the incarnation, or resurrection but from eternity. Jesus said:
I will end it here as this subject has been fully proven by so many theologians through history that I would never be able to add anything to it. John Owen (Quoting St. Jerome) indicates to me that we need not fuss over the term only-begotten. The Messiah, the Son of God has so many names referring to the same 'One and Only' as always believed in the Chruch:
Yes, He was the only begotten Son of God before the incarnation. This has always been one of the core beliefs within both Catholic and Protestant churches, across all the ages.
Hebrews 1:3 speaks of the Son as: 1) the radiance of the eternal light who is the Father himself ( check out: 1 John 1:5) 2) the exact imprint of the being of the Father himself ( check out: John 1:18)
The Father and the Son are co-eval and con-substantial just as the sun is co-eval and con-substantial to its light.
Hebrews 1:5 speaks of the begetting of the Son while Hebrews 1:7 speaks of the creation of the angels.
The Logos was eternally begotten because the begetter is eternal. The angels were created by his powerful rhema in the commencement of creation ( Genesis 2:1,Hebrews 1:2,11:3, Colossians 1:16)
The Logos is the only-begotten Son before creation because no person was begotten of God other than him( John 1:1 compare to Psalm 45:1 LXX, John 1:14 compare to Proverbs 8:21-30 LXX and John 17:5, John 1:18 compare to Psalm 110:3 LXX).
Christians are begotten by the Father in time and in the realm of salvation which is sonship by adoption not by nature ( John 1:12-13,Ephesians 1:5).