While I have never heard a complete treatise on adoptionism, I do believe it is the only Biblically justified position based on my own reading of Scripture.
It cannot be, as it simply
cannot take into account the full revelation of God or of the Son that we are given in Scripture. That's why it was condemned as heresy long ago.
See John 1:12, Colossians 3:12, Galatians 3:26
Are you saying these verses support Adoptionism? How so?
Joh 1:12 But to all who did receive him, who believed in his name, he gave the right to become children of God,
Col 3:12 Put on then, as God's chosen ones, holy and beloved, compassionate hearts, kindness, humility, meekness, and patience,
Gal 3:26 for in Christ Jesus you are all sons of God, through faith.
Do you know what Adoptionism refers to? Humans are clearly said to be adopted as God's children, but that isn't what Adoptionism means. Jesus, or rather the Son, was never adopted--that's rather the whole point of
monogenes, which refers to "one and only" and uniqueness.
Again, as I have pointed out, even Jesus claims he is the only Son:
Joh 3:16 “For God so loved the world, that he gave
his only Son, that whoever believes in him should not perish but have eternal life.
Joh 3:17 For God did not send
his Son into the world to condemn the world, but in order that the world might be saved through him.
Joh 3:18 Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of
the only Son of God.
Verse 18 is interesting because "the name" means the sum of the person--all that he is. Do you disagree with Jesus that he is the only Son of God, that it means something quite different when applied to him than it does when applied to humans?
John seems to have agree with Jesus, perhaps you should too:
Joh 1:14 And the Word became flesh and dwelt among us, and we have seen his glory, glory as of
the only Son from the Father, full of grace and truth.
1Jn 4:9 In this the love of God was made manifest among us, that God sent
his only Son into the world, so that we might live through him.
The NLT translation note says some manuscripts include this. Others (most?) don't and neither does the NLT. To me, it is irrelevant since this notion of Jesus becoming God's son on a certain day is explicit in Hebrews 1 and other places.
If you're going to appeal to a verse, take it in context. First, we see in Heb. 1:2 that "in these last days he has spoken to us
by his Son, whom he appointed the heir of all things,
through whom also he created the world."
How did the Father create the world through his Son if the Son didn't exist when the world was created? Basic reasoning shows that the Son necessarily existed prior to the creation of the world. This comes out again in Heb. 1:10-12:
Heb 1:10 And, “You, Lord, laid the foundation of the earth in the beginning, and the heavens are the work of your hands;
Heb 1:11 they will perish, but you remain; they will all wear out like a garment,
Heb 1:12 like a robe you will roll them up, like a garment they will be changed. But you are the same, and your years will have no end.”
Who is speaking here? The Father, as we can see from verse 8. But look at the astounding claim--he is applying Ps. 102:25-27
to the Son. But who is Ps. 102:25-27 referring to? To Yahweh, to God. The Father is literally saying the Son is also Yahweh. (Yet, we know that there is only one God.) Again, there is a consistency from Heb. 1:2 to 10-12, which is consistent with what John says in 1:1-18.
Whatever you
think Heb. 1:5 states, it
cannot override the clear claims in the other verses--verses 2 and 10-12--or any other verse in the Bible, particularly the NT.
Once again, proof-texting is getting you into trouble.