Charles Ellicott Commentary John 17:21

"that they may all be one; even as thou, Father, [art] in me, and I in thee, that they also may be in us: that the world may believe that thou didst send me." — John 17:21 (ASV)

That they all may be one—that is, both “these” (the Apostles) and them also which shall believe on Me through their word (the whole body of believers in all times and places). He expresses in this grand thought of the unity of the whole Church the fullness of the purpose of His prayer.

As You, Father, are in me, and I in You, that they also may be one in us.—The insertion of “art,” which, as the italics show, is not in the original text, weakens the sense. It is better, therefore, to omit it. The word “one,” in the second clause, is of doubtful authority, and has the appearance of a gloss.

The probable reading, therefore, is, That they all may be one; as You, Father, in Me, and I in You, that they also may be in Us. The meaning is that the union of the Church may be of the same essential nature as that between the Father and the Son. Indeed, this union of the Church may result from the union of individual members with the Father through the Son (John 15:4–10, and others).

The Father is in the Son and the Son is in the Father; both Father and Son take up their abode in the believer, and the believer, therefore, is in the Father and the Son. This is the ideal of the unity of the Church of Christ. If this union with God is realized by each individual, it necessarily follows that all the individuals will be one with each other (Compare to Notes on Ephesians 4:4 and following).

That the world may believe that You have sent me.—The result of the union of believers with God, and therefore with each other, will be that the world will see in it a proof of the divine origin of Christianity, and will believe that the Father sent the Son into the world. As this will be the result, it is thought of as the purpose of the prayer for the whole body of believers. Instances of this result crowd involuntarily upon the mind. The brotherhood of Christians has ever been the witness to their common Fatherhood in God. The divisions of Christendom have ever been the weakness of the Church and the proof to the world that, because they are divided, they cannot be of God (Compare to Note on John 13:35).