I don’t know if this is the best way, but I sure think it would get the conversation going in the right direction if you discussed:
"The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread."
“And I have other sheep, that are not of this fold; I must bring them also, and they will heed my voice. So there shall be one flock, one shepherd.”
Then establish if Jesus built one Church or many churches. Notice that when several churches are mentioned in the Bible, it is evident that the churches are part of a singular Church. Consider, for example, Acts 14:23:
“And when they had appointed elders for them in every** church**, with prayer and fasting they committed them to the Lord in whom they believed.”
This could not be similar to the situation we have today in the Protestant denominations or even non-denominational faith communities. Can you imagine one group of men appointing elders in all the different denominations? If that were so, they would not be different denomination, they would be one church. But they are not. They can’t have another group appoint their elders, because they would not teach their own interpretation of the Bible. The point is, that when “church” is spoken of in the Bible, it has a unity…which is evident by what they believed and who had authority over them.