How about:
John 17:22-23
“The glory which thou hast given to me, I have given to them, that they may be one even as we are one, I in them and thou in me, that they may become perfectly one, so that the world may know that thou hast sent me and hast loved them even as thou hast loved me.”
This passage refers to being visibly united for the world to see. Does Christianity seem united when viewed as so many divided denominations? And if we are to be one, as the Father and Son are One … do we believe that God can disagree within Himself on faith and morals?
also:
Mt. 18:15-18
“If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.”
What’s especially great about this passage is that a couple lines later it also says “wherever two or three are gathered in my name, there am I in the midst of them” which is a pet line for many Protestants, and yet in its context it works entirely against the claim that they want it to (disclaimer: I don’t mean every Protestant so distorts this passage).
The reason this is so great is because in the v15-18 Jesus talks about two or three people getting together in his name to resolve a dispute … and if in that group they cannot yet settle their quarrel they are then to bring the dispute to the church. Yes, Jesus was there amidst those who got together to try and solve a dispute, but might yet still have to refer to the church. Clearly, the presence of Christ in the midst of two or three believers a church does not make! The church is discussed here as a corporate entity, distinct from the invisible unity of believers.