"The Father and I are one" [John 10:30]. This is a very powerful statement by our Lord, occurring as it does in the midst of a conversation with some Jewish leaders who, with feigned sincerity masking an evil purpose, had asked Jesus if he was the Christ. Jesus' response concludes by indicating clearly that he and the Father are one.
As Luther pointed out, the Greek word hen here is in the neuter, not the masculine form, which indicates that it is not a personal oneness that Jesus is talking about, as though he and the Father were one undifferentiated person, but rather it is an ontological oneness that he speaks of: the Father and the Son share the same ground of being, which is divinity or Godhood.
Augustine knew little or no Greek, and thus probably didn't realise the full import of Jesus' words, but he did well to use this text to combat ontological subordinationism. We today might well use it as a basis from which to combat the claims of the Jehovah's Witnesses and others, even misguided Christian theologians who deny the divinity of Christ and his equality of being with the Father.
But most importantly of all, this text can be used to comfort and encourage disconsolate Christians: Jesus and the Father are one, and yet the eternal Son of God, who is one with the Father, the holy one whose death we are responsible for, knows us, knows even our names, has obtained eternal life for us, and promises that no-one shall snatch us out of his hand. Praise be to Him! Amen.