Then the voice which I heard from the sky again is speaking with me and saying, “Go, take the scroll that was being opened in the hand of the angel who stands upon the sea and upon the land,
So I went to the angel, telling him to give me the little scroll. Then he says to me, “Take and eat it up, and it will make your stomach sour, but rather in your mouth it will be as sweet as honey.”
Then I lay hold of the little scroll from the angel’s hand and I devoured it, and it was as sweet as honey in my mouth, but when I ate it, it soured my stomach.
Then they said to me, “You are required again to prophesy on many peoples and ethnicities and languages and sovereigns.”
Revelation 10:8-11
What is the little opened book John was told to eat? How could it taste so good, yet make him sick to his stomach? What John eats he must now prophecy, concerning the whole world, it seems.
Historicist
To the historical expositor, the little opened scroll is the Bible, and represents how the church received the Bible once it was made so publicly available.
For those who had longed for God’s word but were denied it, receiving the scriptures in their own language, made readily accessible by the printing press, was as sweet as honey. But to those who opposed having the Scriptures at the disposal of common folk, the situation was embittered.
By eating the little scroll, John represented the Reformers, who were determined to teach all people how to read the Bible for themselves, and to understand it. In this case, the charge to prophesy was a call to teach and preach, in the spirit of Apostle Paul.
… you have known sacred writings that are able to instruct you for salvation through faith in Christ Jesus. All scripture is inspired by God and is useful for teaching, for reproof, for correction, and for training in righteousness, so that the person of God may be proficient, equipped for every good work.
2 Timothy 3:15-17 (NRSV)
This was a distinctive of the Protestant movement which exchanged a liturgy largely of chants and rites to a worship service of hymns and preaching.

Preterist
Those keeping to the events of 66-70 CE point to a similar episode in Ezekiel’s ministry.
He said to me, “O mortal, eat what is offered to you; eat this scroll, and go, speak to the house of Israel.”
So I opened my mouth, and he gave me the scroll to eat.
He said to me, “Mortal, eat this scroll that I give you and fill your stomach with it.” Then I ate it, and in my mouth it was as sweet as honey.
He said to me, “Mortal, go to the house of Israel and speak my very words to them.
… The spirit lifted me up and bore me away; I went in bitterness in the heat of my spirit, the hand of the Lord being strong upon me.
Ezekiel 3:1-4, 14 (NRSV, emphases mine)
Ezekiel’s commission was to prophecy the destruction of Jerusalem by the Babylonians in 586 BCE. John had a similar commission to prophecy the destruction of Jerusalem by the Romans in 70 CE.
The sweetness came in God answering the prayers of the martyrs, of all those suffering under the persecution of Jewish religious authorities. The bitterness came in Jerusalem’s obdurate refusal to repent and receive their Messiah.
If seen from this viewpoint, then the many languages and peoples represent the Diaspora, the Jewish people spread throughout the Ancient Near East. Though Jerusalem will be the physical center of God’s judgment, it is at the heart of all those of Jewish faith.
On the other hand, some scholars suggest that because John is enjoined to again prophecy, it is now to announce the fall of Rome. The second half of Revelation, then, is not about the destruction of God’s holy city but rather the devastation of the far-flung Roman Empire, comprised as it was of many peoples and ethnicities and languages and sovereigns.
The already opened little scroll is presented as separate and unconnected from the seven-sealed scroll. The angel holding it stands upon the whole known world, and the prophecies it contains have to do with far more than the one nation of Judea, or the one people of God.

Futurist
Commentators who see a future fulfillment do not assign special significance to the scroll except to say the Word of God must first be taken in by the one preaching it. Similarly to historicist interpreters, futurist commentators see the little opened scroll as the Bible, now in the hands of all who would desire to read it.
The simile of eating and digesting God’s Word shows how John must be affected by the scriptures before he can become a minister who will affect others by that same word. The sweetness comes in the promises of God, of God’s grace and love, of the open invitation of the Gospel. The bitterness points to such hard truths such as God’s eternal judgment, and God’s wrath against all wickedness and evil.
Spiritual
There is not agreement as to what it is John must prophecy. Is it the contents of Revelation 11? Or is it the whole second half of John’s Apocalypse, Revelation 12-22? And what makes the book sweet at first, but bitter after time?
Gospel
Perhaps it is the good news of the salvation Jesus brings. The Gospel is beautiful, filled with all good, yet always followed by the bitterness of persecution.
For we are the aroma of Christ to God among those who are being saved and among those who are perishing: to the one group a fragrance from death to death, to the other a fragrance from life to life.
2 Corinthians 2:15-16 (NRSV)
All who experience the Gospel, who both receive and spread the good news, experience both sweetness and suffering.
Whole Counsel of God
John knew the book was sweet to the taste but contained such a heavy and powerful message it would be hard for some to digest. Seen this way, then John was exhorted to deliver the full message. For us today, that might mean quoting the verses that come after John 3:16:
Sweet: “For God so loved the world that he gave his only Son, so that everyone who believes in him may not perish but may have eternal life. Indeed, God did not send the Son into the world to condemn the world but in order that the world might be saved through him. Those who believe in him are not condemned,
Bitter: “but those who do not believe are condemned already because they have not believed in the name of the only Son of God. And this is the judgment, that the light has come into the world, and people loved darkness rather than light because their deeds were evil. For all who do evil hate the light and do not come to the light, so that their deeds may not be exposed.
Sweet: “But those who do what is true come to the light, so that it may be clearly seen that their deeds have been done in God.”
John 3:16 (NRSV, modifications mine)
That John is to prophecy again, speaks to the necessity of giving the rest of God’s revelation. All people must hear this message, every people group and ethnicity, every language group and nationality, both the common folk and the elite. Even the four categories reference the four corners of the earth.

The four perspectives taken from Revelation: Four Views A Parallel Commentary, edited by Steve Gregg