William Barclay gives us an excellent insight into the nature of the true church. He writes:
Suppose a great doctor discovers a cure for cancer. Once that cure is found, it is there. But before it can become available for everyone, it must be taken out to the world. Doctors and surgeons must know about it and be trained to use it. The cure is there, but one person cannot take it out to all the world; a corps of doctors must be the agents whereby it arrives at all the worlds sufferers.
That precisely is what the church is to Jesus Christ. It is in Jesus that all people and all nations can be reconciled to God. But before that can happen, they must know about Jesus Christ, and it is the task of the church to bring that about. Christ is the head; the church is the body. The head must have a body through which it can work. The church is quite literally hands to do Christs work, feet to run upon His errands, and a voice to speak His words.