Anti-Trinitarians commonly cross-reference John 10:30 and John 17:20-23 to try to prove that Jesus and the Father are only "one in purpose", since, as they point out, we cannot be considered one divine being with each other, yet Jesus prayed that we would be one as they are one. This may be the most common objection to the Trinitarian understanding of John 10:30. Let’s look at it.
Jesus, before he became a human being, existed without beginning as God. This is what John 1:1-2 tells us, for example. When creation began, the Word (Jesus) already existed, and he was God. As God, the Word or Son was one with the Father in a way that no human being is or ever can be. Even some anti-Trinitarians acknowledge this point, at least to some extent. For example, Mormons agree that Jesus Christ is part of the one Godhead, with the Father and the Holy Ghost, and most Mormons agree that we will never be part of that Godhead. So, in this sense at least, the Son is one with the Father in a way that we are not and never will be. I will return to this point shortly.
This eternal Word, the Son, then became a human being, known as Jesus Christ. As a human being, Jesus is now one of us. As a human being, Jesus’ oneness with the Father is a unity that no other human beings naturally have, but that Jesus offers to share with us by grace. This is not the same "oneness" that Jesus had (and still has) as God with the Father, but it has its source in that oneness. In effect, Jesus has two kinds or modes of oneness with the Father: the divine oneness with the Father that he has always had by virtue of being God, and a derived oneness with the Father that he enjoys as a perfect human being. (He can have the derived oneness with the Father only because he already had the divine oneness.) Jesus cannot share his divine oneness with us, but he can share his derived oneness with us. As I will argue below, this derived oneness is in a sense part of the Son’s divine oneness with the Father. It is not an entirely new oneness, but it is possible because the Son, who was already one with the Father, came so that we might experience that aspect of their oneness that creatures can share.
We see these two kinds of oneness in John 10 and John 17. Let’s take each passage one at a time.
I have commented on John 10:30 in context in depth, so I can be brief here. Recall that in John 10, Jesus lays claim to divine titles and functions that are unique to God. He is "the good shepherd" (vv. 11, 14), like "the LORD is my shepherd" in Psalm 23:1. The people of God in Israel, Jesus says, are his sheep, even though the Old Testament says they are God’s sheep (Ps. 100:3). Jesus says that he gives his sheep, his people, eternal life (v. 28); again, this is a prerogative of deity, not something you or I can ever claim to do. Jesus says that no one will be able to snatch his people out of his hand (v. 28 again). This claim by Jesus to give life to whomever he chooses, and that no one can snatch them out of his hand, echoes what the LORD God said in the Old Testament: "See now that I, even I, am he; there is no god besides me. I kill and I make alive; I wound and I heal; and no one can deliver from my hand" (Deut. 32:39 NRSV; see also Is. 43:13). I think Mormons would have to agree with this, because they agree that Jesus was Jehovah, the LORD God of the Old Testament. Other anti-Trinitarians will balk at this point, but they can hardly deny that what Jesus is saying here about himself is not something that could be said about any of us. We do not have this sort of power over life and death.
Jesus then says that no one can snatch the sheep out of his Father’s hand, either (v. 29). This statement makes it clear that the imagery of sheep that no one can snatch from his hand is a way of speaking of Christ’s divine power in salvation. It is in this context that Jesus says, â€œI and the Father are one (John 10:30). Clearly, in this context, Jesus is talking about the unity that he has with the Father in their exercise of divine power in salvation of the sheep. This is not merely being fully in agreement with the Father (one in purpose ). It is a oneness that Jesus has with the Father because he is the LORD Jehovah, truly God, and therefore this is a oneness that Jesus cannot share with us.
Now let’s look at John 17:
"I ask not only on behalf of these, but also on behalf of those who will believe in me through their word, that they may all be one. As you, Father, 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. The glory that you have given me I have given them, so that they may be one, as we are one, I in them and you in me, that they may become completely one, so that the world may know that you have sent me and have loved them even as you have loved me" (John 17:20-23 NRSV).
The first thing to notice here is that Jesus prays that his disciples will be one â€œso that the world may knowâ€ that the Father sent Jesus and that he loves them just as he loved Jesus (v. 23). That means that Jesus is praying for a unity that is at least possible to realize here in our mortality. It obviously won’t do the world any good, as far as showing them that the Father sent Jesus, if we can’t be one in this sense until after we rise from the dead and become exalted in heaven. Jesus must, therefore, be talking about a way that human beings can be united now, while still mortal beings.
The second thing to notice is that the way this unity is expressed is in love (see also vv. 24-26). It is the love of Christians one for another that will convince the world that the Father sent Jesus (see also John 13:34-35). Jesus is obviously not talking about us becoming one with God in the sense of gaining divine powers; he is talking about us loving each other the way the Father and Jesus love each other. That is a "oneness" that Jesus can share with us. Obviously, Jesus and the Father were one in this sense long before Jesus became a human being; it is therefore, in a sense, part of the oneness that Jesus and the Father have always had as God. It is the aspect of the divine oneness that can be shared with those who are not divine.
In conclusion, John 17 does not prove that the Father and Jesus are one only in purpose. John 17 is not even talking about being one in purpose. It is talking about being one in the love that the Father and Jesus have for each other. That love is part of the divine oneness that the Father and Jesus have shared for eternity as God. Insofar as that divine unity is also a unity of divine power and prerogatives, it is a divine oneness that we will never have.