He does not say, That I and they maybe one, though He might have said so in the sense, that He was the head of the Church, and the Church His body; not one thing, but one person: the head and the body being one Christ. But shewing something else, viz. that His divinity is consubstantial with the Father, He prays that His people may in like manner be one; but one in Christ, not only by the same nature, in which mortal man is made equal to the Angels, but also by the same will, agreeing most entirely in the same mind, and melted into one Spirit by the fire of love. This is the meaning of, That they may be one as We are: viz. that as the Father and the Son are one not only by equality of substance, but also in will, so they, between whom and God the Son is Mediator, may be one not only by the union of nature, but by the union of love.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: iv. de Trin. c. ix