Taken from The Church and the Last Things:
Now here, surely, certain things can be said without any fear of contradiction. If we are to be guided by the scriptural teaching, then we must agree at once that the unity that the Scripture is interested in is spiritual unity. How often John 17 is misquoted! People just tear a phrase right out of its context. ‘That they all may be one,’ they say quoting verse 12, and they leave it at that. They insist also that division in the Church is the greatest sin of all. Now, of course, we all agree that division is regrettable; schism is certainly sin. Yes, but when that is interpreted as meaning that anybody who calls himself a Christian, no matter what the shape or form, is someone with whom we would be in absolute unity in every respect, then that is a contradiction of what John 17 teaches.
John 17 surely makes the character of this unity quite plain and clear. Our Lord’s terms are these: ‘As thou, Father, are in me, and I in thee, that they also may be one in us… And the glory which thou gavest me I have given them; that they may be one, even as we are one’ (vv. 21-22). That is all spiritual. Our Lord is talking of the relationship between the Father and the Son, and those who are in Christ, who are in the Father and the Son, and He has already told us certain things about these people. He says, ‘For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out of thee, and they have believed that thou didst send me’ (v.8). So our Lord’s words about unity are only applicable to people who believe that particular doctrine, and if people tell me that they are Christians but say that Jesus was only a man, then I have no unity with them. I do not belong to them. They may call themselves Christians, but if they have not believed and accepted this, there is no basis for unity. It is spiritual unity.
It always helps to look as Scripture in context.