I found a good answer -in a footnote to a classic statement about the basis of our Christian union… 


 AUTHORS NOTES from The Sole Test of Communion” by Roland Allen

1) Perhaps someone will say that all who were received into the Church by the Apostles accepted the apostolic doctrine and order, none were admitted   who did not accept these, and consequently there is here another test of communion.


To this I would answer: FIRST, the whole point of the story of Cornelius and the admission of the Gentiles lies in the fact that these people had not accepted what up to that moment had been considered a necessary part of the Christian teaching. The question was whether they could be admitted without accepting the teaching and undergoing the rite. It was that question which was settled by the acknowledgment that they had received the Holy Spirit. 

SECOND, when the Apostles spoke of men with whom they were not in communion, they used language which showed that they were convinced that those with whom they were not in communion had not the Spirit (Jude 19). The moment it was admitted that they had the Spirit they were accepted.

The difficulty today is that Christians acknowledge that others have the Spirit, and yet do not recognize that they ought to be, and must be, because spiritually they are, in communion with one another. Those who hold a theory of the Church which excludes from communion those whom they admit to have the Spirit of Christ, simply proclaim that their theory is in flat contradiction to the spiritual reality. The Spirit accepts them and dwells in them; the theory excludes them. 

We must distinguish carefully tests which prove whether the Spirit is given like Paul’s ” No man can say that Jesus is Lord,, but by the Holy Ghost” (I Cor.12.3), and tests which are applied after it is admitted that the Holy Ghost is given. The first is a true test, for there can be no communion between those who have and those who have not the Spirit of Christ. The second is the introduction of a test that subverts a spiritual fact already recognized.  

2) It will perhaps be said that in our present state of schism this assertion of spiritual principle can give us no definite guidance for action, can provide us with no clear programme, and must remain unfruitful. Surely that is not wholly true. It certainly must help us if we recognize that it is the presence of the Holy Spirit which creates a unity which we can never create. If men believe in the existence of this unity , they may begin to desire it, to seek for it, and seeking it to find it. If, when they find it, they refuse to deny it, in due time, by ways now unsearchable, they will surely return to visible communion. 

 It would make a difference if Christians, in their approach to one another realized that, in spite of appearances, they were in fact one. If, in their seeking after reunion, they realized that they were seeking not to create a unity which does not yet exist, but to find an expression for a unity which does exist, which is indeed the one elemental reality, they would approach one another in a better frame of mind. _________________________________________________________Roland Allen (1868-1947) was an Anglican missionary in China working with the Society of the Propagation of the Gospel. Later he labored some 40 years writing missionary principles, retiring to Africa, where he died in Kenya in 1947. First edition of Pentecost and The World was in 1912. Complete article here – The SOLE TEST of COMMUNION