Go to the next verse and we read "Then He commanded His Disciples that they should tell no one that He was Jesus the Christ."
So He was talking to His group of Disciples. That's why they stop at vs. 19. They don't want you to figure that one out.
You must read scripture in the context of how it is understood and was understood by the early disciples. Otherwise, this becomes a sophomoric exercise.
I’ll take the interpretation of the early Church Fathers, some of whom were contemporaries of the Evangelist John than the shallow stuff of the Billy Grahams, the Moonies, and the Joel Osteens of our day.
John 21: 25
“But there are also many other things which Jesus did; which, if they were written every one, the world itself, I think, would not be able to contain the books that should be written.”
Here’s St. Irenaeus:
“It is possible, then, for everyone in every Church, who may wish to know the truth, to contemplate the tradition of the apostles which has been made known to us throughout the whole world. And we are in a position to enumerate those who were instituted bishops by the apostles and their successors down to our own times, men who neither knew nor taught anything like what these heretics rave about” (Against Heresies 3:3:1 [A.D. 189]).
In the year 110 A.D., barely fifteen years after the book of Revelation was written, while on his way to execution St. Ignatius of Antioch wrote: “Where the bishop is present, there let the congregation gather, just as where Jesus Christ is, there is the Catholic church”. The Church believes that when the bishops speak as teachers, Christ speaks; for he said to them: “He who hears you, hears me; and he who rejects you, rejects me” (Lk 10, 16).
This belief by a litany of saints, martyrs, and stigmatists in the ONE Church established by Petrine authority is why we have entire constellations of intellectual non-Catholics from every religious denomination convert to Catholicism.