From Evangelical Theology pgs 174-177:
What is the Church? There is one thing certain about it: the Church has a great many attributes, but that which is absolutely essential is its absolute unity. There is no doubt if there be but one God, there is but one Church; if there be but one Christ, there is but one Church; if there be but one Holy Ghost, there is but one Church. This is absolutely settled—there is but one Church. We have heard about the visible and invisible Church, as if there were two churches. There cannot be two churches, one that is visible and another that is invisible. There is but one Church, and that Church is visible or invisible just according to the eye that is looking, just according to the point of view taken….
There have been two distinct conceptions of the Church: one is the theory that the Church consists of an organized society which God has constituted, that identity consists in its external form as well as in its spirit, and that its life depends upon the continuity of officers from generation to generation. This is held by a great many able men, men of intellect, and by many respectable, level-headed Christians as well.
I hold this to be simply impossible. The marks of the Church are catholicity, apostolicity, infallibility, and purity. Now, apply that to any corporation—to the Church in Jerusalem or to the Church in Antioch; to the Congregational Church, to the Presbyterian, or to the Prelatical Churches. I do not care as to the form; but there never did exist, and there does not now exist, any organized society upon the face of the earth of which these qualities could be predicated. Not one of these societies has apostolicity—that is, precisely the apostolic form as well as the apostolic spirit; not one of these societies has had an absolute organic continuity, or has, without modification, preserved it. Societies, like the Church of Rome, which are most conspicuous in claiming these marks for themselves, are most conspicuously unworthy of them, because there is no comparison between their ritual of service, their organization, and the apostolic Church with which they claim to be identified.
The only possible definition of a Church is that it consists of what is termed "the body of Christ"—that is, human souls regenerated by the presence and power of the Holy Ghost, kept in immediate union with Christ. Of this you can predicate apostostolicity, catholicity, and the sanctifying power and perpetual presence of the Holy Ghost, which belongs to the Church of Christ. This is the true Church, which exists through all the successive generations of men, which is united to Christ, and which shares in the benefits of his redemption through the indwelling of the Holy Ghost. This great body is one because the Holy Ghost dwells in it and makes it one. This Church is apostolical, because it is unchanging as to apostolic doctrine; it is catholic, because it contains in one body all of God's people in all worlds and in all time; it unites all from the creation of the world to the coming of Christ, and all from the coming of Christ to the end of the world in one body—absolutely one, both visible and invisible.…
Now, as to the unity of this Church I have something to say. A great many are agitated at present with regard to Church unity and its manifestations, and I think there is a great deal of confusion of thought as to the original conception of the Church itself. If the Church be an external society, then all deviation from that society is of the nature of schism; but if the Church be in its essence a great spiritual body, constituted by the indwelling of the Holy Ghost through all the ages and nations, uniting all to Christ, and if its external organization is only accidental and temporary, and subject to change and variation, then deviation of organization, unless touched by the spirit of schism, is not detrimental to the Church. I do believe that God's purpose, on the contrary, has been to differentiate his Church without end. You know that the very highest form of beauty of which you can conceive, the very highest form of order, is multiplicity in unity, and unity in multiplicity; the higher the order of unity, the greater must be the multiplicity.
This is so everywhere. Go to the ocean: every drop of water is the repetition of every other drop, and there is union simply without diversity. Go to the desert of Sahara, and every grain of sand is the duplicate of every other grain of sand; but there is no unity, no life.
You could not make a great cathedral by piling up simple identical rhomboids or cubes of stone. It is because you differentiate, and make every stone of a different form in order to perform a different function, and then build them up out of this multitudinous origination into the continuity and unity of the one plan or architectural idea, that you have your cathedral. You could not make a great piece of music simply by multiplying the same tone or sound. In order to obtain the harmony of a great orchestra, you get together a large number of musical instruments, or you have a great number of human voices in a choir, and you combine them; then you have an infinite variety of quality and infinite variety of tone. You combine them in the absolute unit of the one great musical idea which you seek to express.
But if this is true of such things, it is more true of Christ's Church. If God had followed our idea, how simple a thing it would have been to make a united Church descending from Adam and Eve! We might think that was all that could be done, and there would be then no stones of stumbling. You could then watch this Church, and it would go on indefinitely and without limit.
Now, what has God been doing? He has broken humanity up into infinite varieties. This has been his method. He has been driving it into every clime. He has been driving it into every age through the succession of centuries. He has been moulding human nature under every variety of influences through all time, until he has got men in every age, every tribe, every tongue, every nation, every colour, every fashion—in order to do what? Simply to build up a variety, to build up the rich, inexhaustible variety which constitutes the beauty in unity of this great infinite Church of the first-born, whose final dwelling-place is to be in heaven.