See commentary on John 17:20-23.

John 17:21

_ _ that they all may be one, as thou, Father, art in me, and I in thee, that they may be one in us — The indwelling Spirit of the Father and the Son is the one perfect bond of union, knitting up into a living unity, first all believers amongst themselves; next, this unity into one still higher, with the Father and the Son. (Observe, that Christ never mixes Himself up with His disciples as He associates Himself with the Father, but says I in THEM and THEY in US).

_ _ that the world may believe that thou hast sent me — sentest me. So the grand impression upon the world at large, that the mission of Christ is divine, is to be made by the unity of His disciples. Of course, then, it must be something that shall be visible or perceptible to the world. What is it, then? Not certainly a merely formal, mechanical unity of ecclesiastical machinery. For as that may, and to a large extent does, exist in both the Western and Eastern churches, with little of the Spirit of Christ, yea much, much with which the Spirit of Christ cannot dwell so instead of convincing the world beyond its own pale of the divinity of the Gospel, it generates infidelity to a large extent within its own bosom. But the Spirit of Christ, illuminating, transforming, and reigning in the hearts of the genuine disciples of Christ, drawing them to each other as members of one family, and prompting them to loving co-operation for the good of the world — this is what, when sufficiently glowing and extended, shall force conviction upon the world that Christianity is divine. Doubtless, the more that differences among Christians disappear — the more they can agree even in minor matters — the impression upon the world may be expected to be greater. But it is not dependent upon this; for living and loving oneness in Christ is sometimes more touchingly seen even amidst and in spite of minor differences, than where no such differences exist to try the strength of their deeper unity. Yet till this living brotherhood in Christ shall show itself strong enough to destroy the sectarianism, selfishness, carnality, and apathy that eat out the heart of Christianity in all the visible sections of it, in vain shall we expect the world to be overawed by it. It is when “the Spirit shall be poured upon us from on high,” as a Spirit of truth and love, and upon all parts of the Christian territory alike, melting down differences and heart burnings, kindling astonishment and shame at past unfruitfulness, drawing forth longings of catholic affection, and yearnings over a world lying in wickedness, embodying themselves in palpable forms and active measures — it is then that we may expect the effect here announced to be produced, and then it will be irresistible. Should not Christians ponder these things? Should not the same mind be in them which was also in Christ Jesus about this matter? Should not His prayer be theirs?