John 17:21

21.That all may be one. He again lays down the end of our happiness as consisting in unity, and justly; for the ruin of the human race is, that, having been alienated from God, it is also broken and scattered in itself. The restoration of it, therefore, on the contrary, consists in its being properly united in one body, as Paul declares the perfection of the Church to consist in

believers being joined together in one spirit and says that apostles, prophets, evangelists, and pastors, were given, that they might edify and restore the body of Christ, till it came to the unity of faith; and therefore he exhorts believers to grow into Christ, who is the Head, from whom the whole body joined together, and connected by every bond of supply, according to the operation in the measure of every part, maketh increase of it to edifcation,
(Ephesians 4:3.)

Wherefore, whenever Christ speaks about unity, let us remember how basely and shockingly, when separated from him, the world is scattered; and, next, let us learn that the commencement of a blessed life is, that we be all governed, and that we all live, by the Spirit of Christ alone.

Again, it ought to be understood, that, in every instance in which Christ declares, in this chapter, that he is one with the Father, he does not speak simply of his Divine essence, but that he is called one as regards his mediatorial office, and in so far as he is our Head. Many of the fathers, no doubt, interpreted these words as meaning, absolutely, that Christ is one with the Father, because he is the eternal God. But their dispute with the Arians led them to seize on detached passages, and to torture them out of their natural meaning, in order to employ them against their antagonists. (124) Now, Christ’s design was widely different from that of raising our minds to a mere speculation about his hidden Divinity; for he reasons from the end, by showing that we ought to be one, otherwise the unity which he has with the Father would be fruitless and unavailing. To comprehend aright what was intended by saying, that Christ and the Father are one, we must take care not to deprive Christ of his office as Mediator, but must rather view him as he is the Head of the Church, and unite him with his members. Thus will the chain of thought be preserved, that, in order to prevent the unity of the Son with the Father from being fruitless and unavailing, the power of that unity must be diffused through the whole body of believers. Hence, too, we infer that we are one with the Son of God; (125) not because he conveys his substance to us, but because, by the power of his Spirit, he imparts to us his life and all the blessings which he has received from the Father.

That the world may believe. Some explain the word world to mean the elect, who, at that time, were still dispersed; but since the word world, throughout the whole of this chapter, denotes the reprobate, I am more inclined to adopt a different opinion. It happens that, immediately afterwards, he draws a distinction between all his people and the same world which he now mentions.

The verb, to believe, has been inaccurately used by the Evangelist for the verb, to know; that is, when unbelievers, convinced by their own experienc, perceive the heavenly and Divine glory of Christ. The consequence is, that, believing, they do not believe, because this conviction does not penetrate into the inward feeling of the heart. And it is a just vengeance of God, that the splendor of Divine glory dazzles the eyes of the reprobate because they do not deserve to have a clear and pure view of it. He afterwards uses the verb, to know in the same sense.