It is very tempting to start with the answer, the day, the only day that makes any sense in light of all the Scripture verses, is day 1
Psalm 2
“I will tell of the decree: The Lord said to me, “You are my Son; today I have begotten you.”
Psalm 2:7
Notice that the Son is retelling the decree which He witnessed the Father speaking to Him. This is a retelling, not a prophecy for the future. This is past tense and it’s in the psalms, long before His incarnation. It’s not the Lord “will tell”. That means He was already Son then.
He was fully aware of the decree. And He was part of that begetting. If He was born a human 4900 years later, what then was He begotten as on this occasion?
Creator
Paul in Hebrews shows us that the Son was the Creator through which all things were made
“but 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.”
Hebrews 1:2
This last part, “through whom he created the world” sets a limitation. The limitation is, namely, that the Son had to have been begotten before the world was created. Otherwise the world was not made through the Son.
And we know that the world(eons/ages) were made on day one according to Genesis 1:1
“In the beginning, God created the heavens and the earth. The earth was without form and void, and darkness was over the face of the deep. And the Spirit of God was hovering over the face of the waters. And God said, “Let there be light,” and there was light. And God saw that the light was good. And God separated the light from the darkness. God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.”
Genesis 1:1-5
The ages started on day 1.
Jesus before Creation
“And now, Father, glorify me in your own presence with the glory that I had with you before the world existed.
Father, I desire that they also, whom you have given me, may be with me where I am, to see my glory that you have given me because you loved me before the foundation of the world.”
John 17:5, 24
According to the Scriptures ALL things were made through, Him and for Him
“For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him.”
Colossians 1:16
Absolutely all things were made through Him and for Him
“All things were made through him, and without him was not any thing made that was made.”
John 1:3
This is important to take into account that nothing existed outside of Him that is created. Because in order to understand which day, we must pinpoint when He first came into the picture. And the text says He had to have existed before anything was even created.
This is contrasted to angels who do not have the title of God or creator
“But of the Son he says, “Your throne, O God, is forever and ever, the scepter of uprightness is the scepter of your kingdom. You have loved righteousness and hated wickedness; therefore God, your God, has anointed you with the oil of gladness beyond your companions.” And, “You, Lord, laid the foundation of the earth in the beginning, and the heavens are the work of your hands; they will perish, but you remain; they will all wear out like a garment, 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.””
Hebrews 1:8-12
We also understand from Jesus that prior to Creation He had a glory that he lacked after creation, a glory He wanted back.
Who was Jesus prior to incarnation?
“For to which of the angels did God ever say, “You are my Son, today I have begotten you”? Or again, “I will be to him a father, and he shall be to me a son”?”
Hebrews 1:5
He was the Angel of the Lord and God Himself
“In the beginning was the Word, and the Word was with God, and the Word was God.”
John 1:1
Notice how the OT makes this superimposed point
“And the angel of the Lord appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed.”
Exodus 3:2
“When the Lord saw that he turned aside to see, God called to him out of the bush, “Moses, Moses!” And he said, “Here I am.””
Exodus 3:4
The Angel of the Lord was in the bush and two verses later the Angel is being referred to as God still in the bush.
What happened? How is it possible that God would be an Angel?
Kenosis
We already know that Jesus took on a human body and yet remained God, despite nullifying Voluntarily His divine attributes.
“Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, by taking the form of a servant, being born in the likeness of men.”
Philippians 2:5-7
Piecing it together. Jesus took on a human body, but John 17 says He existed with God prior to Creation. So when did He take on the body of the Angel of the Lord?
“I will tell of the decree: The Lord said to me, “You are my Son; today I have begotten you.”
Psalm 2:7
When He was begotten on day one of Creation prior to creating anything and after He took off the glory He had prior to Creating everything.
(It’s worthy to note that Lucifer seeing the Angel of the Lord looked identical to himself, thought it worthy to grasp at equality with God, the visible God the OT, the Angel of the Lord. He said this much to Eve
“For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.””
Genesis 3:5
Lucifer seemed persuaded that he too could be God if this archangel could be, why not also he? From the heart spoke Lucifer)
The exact day
Which day was this referring to? The first day of Creation, when Jesus was begotten and given a heavenly body.
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Bill Porter’s challenge
“Long ago, at many times and in many ways, God spoke to our fathers by the prophets,”
Hebrews 1:1
That is true, in the OT God spoke to the prophets and the prophets spoke to the people on God’s behalf. The prophets would receive the oracles and pass them onto the people.
“but 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.”
Hebrews 1:2
But Bill Porter says God spoke through His Son, now that He incarnated. Firstly that last part is not in the text. That doesn’t make sense because if the Son only came to be begotten at incarnation, how did God create the world through the Son? He couldn’t have. According to Bill Porter and others, the Son was begotten at incarnation. Well how then did God create the world through the Son who didn’t exist yet? The text clearly says it was the Son, not the spirit of the Son, not the Word but the Son.
Stephen points out that an angel spoke to Moses
““Now when forty years had passed, an angel appeared to him in the wilderness of Mount Sinai, in a flame of fire in a bush.”
Acts 7:30
He goes on to say
“This is the one who was in the congregation in the wilderness with the angel who spoke to him at Mount Sinai, and with our fathers. He received living oracles to give to us.”
Acts 7:38
Moses received oracle from the Angel but the OT says it was from God and God was on the mountain and God said you can only see my backside, and the people said they didn’t want to hear God speak to them any longer.
“For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh,”
Romans 8:3
God didn’t say He sent the Word, He said He sent His SON. But if the Son was begotten after conception how could He send Him? Obviously the Son had to exist before being sent.
Additional remark
Why was it even necessary for the Son to take on a heavenly body? Why not remain a spirit?
God is spirit. The Son is spirit and the Son is God but the Son when He was begotten at the beginning of creation took on a heavenly body. Heavenly bodies are supernatural bodies. They interact with the natural world but are able to exceed the natural laws and are suited for the heavenly realm too.
In order to interact with the natural and the supernatural world God decided to use vessels or bodies. Had the Son remained a spirit His interaction would have been somewhat limited.
In order to speak as a human, you need a human body with functional organs and a mind that can coherently speak. If someone is just a spirit without either a heavenly or earthly body then it cannot interact with the physical world. Humans do not interact “naturally” with the supernatural. Humans have added limitations. Humans require a means of intercepting, deciphering spiritual information. If the information is written down, we can read it but having interactions with the spiritual is limited and requires a lot of self-denial as well as devoted dedication. The best way to do it is if God takes on a human body. It’s simplest. And He did do that. He then gave the begotten Son a human body, so that God didn’t need to speak to prophets who committed themselves to hearing and seeing God but now God could interact directly with humans on the natural/material level.
Hence
“Long ago, at many times and in many ways, God spoke to our fathers by the prophets,”
but 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.”
Hebrews 1:1,2
The plan appears to have been from the beginning to take the Son and let Him be the visible God in Creation while the Father remained invisible. And the Father remained invisible throughout history partly by choice but partly because He didn’t take one a housing of His own.
The Father could, like with believing Christians make His residence in their human vessel. In the same way the Father was in the Son. But the Son was also in the Father (haven’t nailed how to articulate this exactly yet).
The design of Creation is incredibly sophisticated, strategically planned and thought out and on multiple levels, natural, supernatural/aka spiritual and God level.
It’s remarkable how deliberate everything was done. And how each level has its own laws and how the levels interact between each other.