I was a baby, now I’m an adult. I was an adolescent, and will be (God willing) a senior citizen. There are certain things in me that change, and others that stay the same. Scholastic philosophy calls this the difference between what is accidental and essential. What is essential remains the same from my conception to death. Everything else is constantly changing. I am essentially the same person from my conception, which is what my first name represents. I am that person before I’m given the name, and all the way through to my death. What I call “I” never becomes different, although maturity and experience produce accidental changes that “I” live through along the way.

The same principle applies to the Church as the body of Christ. Along with its divine Head, it makes up the mystical Person of Christ. From its conception in the calling of the 12 apostles and its birth on Pentecost, through to the second coming at the end of history, the Church remains essentially the same. One identity. One organism. One family, established by its Head. All of its non-essentials through the centuries can change, like the form of liturgy celebrated, how popes are chosen, the emphases it gives certain doctrines at various times in history, etc. These are non-essential and changeable. But identity, essential form, faith, morals, and sacraments necessary for salvation cannot change this side of heaven.

The Church is one, as Christ conceived it. That is why He established a hierarchy and so fervently prayed in John 17 that, in its fallen state, the mystical body of Christ would continue to act as one.