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.

— Jamieson-Fausset-Brown Commentary (public domain)