The Church is the body of Christ
The best way to understand that the Church is not just an institution is to understand it as the body of Christ. That’s the way St. Paul understood the Church, and that’s the way we have it in sacred scripture. Our Blessed Lord says all through the gospels that he is going to establish a new body, a new qahal, a new people of God. After all, when people are united for a given purpose, they are a body. Now our Lord did not use the word “body” precisely because his own physical body was before everyone. He used the word “kingdom” because that was a word the Jews could understand. But when St. Paul was talking to the pagans, he had to use a word which was more understandable, namely, “body.” Our Lord communicated exactly the same idea. He said that the new people he would unite with himself would be related to him as branches and vine. He said, “I am the vine, you are the branches.” The truth that he had, he said he would give to them. “My truth I give to you. My power I give you.” Also he communicated the power to forgive sins. Our Blessed Lord said that he would develop and form a new body which would be very small at first like a mustard seed, and then it would grow and spread throughout the entire world.