It is normally understood that we share the divine nature because Jesus had that divine nature with the authority to share it with us. So there is the giver and there are those who receive the gift.
It is the oneness of God with the Father that gives him the authority to share with us the divine nature that enables us to share in the divine nature, though not in the divine identity. That goes without saying.
It is understood and presumed in the saying about oneness that we share with God. God has to give it to us. And God is doing it through His only Son.
He didn't have to expound upon the difference between divine Messiah and human recipients of God's grace. It was understood and presupposed.
If we receive something by grace, we are purely humans. If it is given by the Son of God he must be the divine source of this gift.
No, there need be no addendum to explain that the divine Son must give, by grace, to strictly humans who receive that gift. The interesting facet of this is that God, in giving us His grace, chose to do so through the appearance of His Son, Jesus. Jesus, as such, was the divine agent of this gift of a divine nature to people who accept the conditions upon which we receive it.
The fact we receive the same Spirit that empowered Jesus only indicates we have the same God and the same nature. This does not imply that the oneness Jesus had with his Father is the same as the oneness we have with God and Jesus. As another commentator describes it, it is similtude, and not equality.
We have no disagreement about our sharing in the same Spirit as Jesus did, thus displaying the same virtues and character that Jesus did. The question is about "oneness," which I believe inferred an essential difference at the same time it recommended an essential sameness.
One draws general conclusions when familiar with the NT Scriptures overall. Jesus knew the OT Scriptures, of course, and recognized that the Law provided sacrifices for all Israel, and not just for the most wicked. All are contaminated by sin, and all therefore require an atonement for sin if they wish to enjoy fellowship with God and receive from Him eternal salvation.
So that's the purpose Jesus came, to provide this sacrifice of atonement so that all may receive Eternal Life, conditioned upon their acceptance of the conditions. I need not explain how this fits into what Jesus said about "oneness" because it is background information and already understood as part of the Messianic mission.
The difference between Jesus' unity with God from our unity with God is likewise understood in the difference between unblemished Messiah, the Son of God, and mankind overall who are sinners and in need of divine grace. This should not have to be explored and understood.
So look at the conversation and recognize that more is being said than just "unity." There is the matter of what the "unity" entails, and what it provides.
The unity entails Jesus as the Messianic atonement, and it provides a means for fallen men to be redeemed, in order to enjoy an eternal relationship with God. I will underline some of this in the following verses...
John 17.11 I will remain in the world no longer, but they are still in the world, and I am coming to you. Holy Father, protect them by the power of your name, the name you gave me, so that they may be one as we are one. 12 While I was with them, I protected them and kept them safe by that name you gave me. None has been lost except the one doomed to destruction so that Scripture would be fulfilled.
13 “I am coming to you now, but I say these things while I am still in the world, so that they may have the full measure of my joy within them. 14 I have given them your word and the world has hated them, for they are not of the world any more than I am of the world. 15 My prayer is not that you take them out of the world but that you protect them from the evil one. 16 They are not of the world, even as I am not of it. 17 Sanctify them by the truth; your word is truth. 18 As you sent me into the world, I have sent them into the world. 19 For them I sanctify myself, that they too may be truly sanctified.
20 “My prayer is not for them alone. I pray also for those who will believe in me through their message, 21 that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. 22 I have given them the glory that you gave me, that they may be one as we are one— 23 I in them and you in me—so that they may be brought to complete unity. Then the world will know that you sent me and have loved them even as you have loved me.
1) Jesus has the name initially--he is Deity. He is Messiah in order to share God's name, and the authority it brings to redeem men.
2) Jesus has the fulness of Divine joy. As Messiah he wishes to impart that joy to his Disciples.
3) He has come into the world as Messiah to display holiness in mankind. He had that initially and wishes to impart that to us by giving us of his own Spirit.
4) Jesus as Messiah owns the basis of spiritual unity with God, which is eternal unity. As humans we achieve this by grace progressively, as we repent, convert, and manifest his righteousness through our obedience.
In all cases it is presumed that Jesus is a Messiah in complete unity with God's Person. It is presumed that he became man as part of God's plan to give His grace to us through the conduit or agency of His Son.
Taking the word "unity" or "oneness" in simplicity, or outside of its context, we may only see our participation in God as Jesus participated in God. But looking at it more closely in context, it is clear that the assumption is being made that the Messiah has a different kind of unity with God, denoting equality, even though he is drawing us into a common spiritual unity.