Revelation 10: 4, 8-11
And when the seven thunders had uttered their voices, I was about to write: and I heard a voice from heaven saying unto me, Seal up those things which the seven thunders uttered, and write them not.
And the voice which I heard from heaven spake unto me again and said, Go, and take the little book which is open in the hand of the angel which standeth upon the sea and upon the earth.
And I went unto the angel, and said unto him, give me the little book. And he said unto me, Take it, and eat it up; and it shall make thy belly bitter but it shall be in thy mouth sweet as honey.
And I took the little book out of the angel’s hand, and ate it up; and it was in my mouth sweet as honey: and as soon as I had eaten it my belly was bitter.
And he said unto me, Thou must prophesy again before many peoples, and nations, and tongues, and kings.
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Many have wondered what the book that St. John swallowed is. He is told not to write down what the seven thunders said, and then he is told to swallow the little book. The little book is the entire content of the book that St. John was about to write, after the vision, the Book of Revelation (also known as the Book of Apocalypse.) Remember, St. John was in this vision, he was not taking actual notes. He had a vision of himself about to take notes when he is told not to. What he is given instead is the entire memory of what would be permitted for him to write. In other words he ate the advance copy of the book he was about to write based on the vision that he was in the middle of.
This is one reason why it is a “little” book, for it would be just one book, albeit the last one, of the Bible. Second, that is why it is sweet like honey, as it is the true revelation of God, but bitter in the stomach, because there is also a hard message in it for the world of the impending judgment. Third that is why after swallowing he is told that he must prophesy again before many peoples, nations, tongues, and kings, even though he was already an old man when this occurred. The angel was telling him that this book that he would write would be published and translated into many languages to many peoples all around the world.