The little book was very pleasing to John's taste. Such is the way we all approach the Bible. We love to study it and find new things (Jeremiah 15:16). It is very pleasing, even thrilling, going down, but the consequences of eating it are sometimes quite unsettling once we understand what it is telling us to do. It can be even sickening in the way that it turns our world upside-down.
This "tasting good" can also be like our first love—when we are all zealous for God's truth. Then, as we come to understand what it really means and how it affects our life, it becomes less and less sweet and more and more bitter. It makes us do things that are wrenching to our lives. This is where the bitterness comes in. Our human nature often does not want to do the things that it tells us we must do, which causes upset, pain, even personal calamity. Sometimes we have to go against a family member who is perhaps staunchly religious but of a different belief, who does not like what we are doing. It might cause the disruption of a family, the loss of a job, even persecution and death. That is how bitter following the Word of God can be. How bitter is death?
It is likely that this phrase—his stomach became bitter—means that it did not just become queasy. John threw up. He fell to the ground and threw up everything he ate. A little later, it says that the angel tells him, "John, get up. Rise." It is in the record to show us the normal reaction to God's Word, especially to the prophet or apostle who is commissioned to preach it. God is working with symbols, behaviors, and starkly contrasting things. The phraseology suggest a suddenness: John loved it as it went down, but it hit his stomach and came right back up again. As in Ezekiel (there are a number of parallels between John's experience in Revelation and the prophet's in Ezekiel), God worked with a man's behavior. God wants us to see how wrenching taking God's Word into us is to a person's life. We come out of a world that is totally opposite of His way. And when these two ways of life meet—like vinegar and soda—it causes a sometimes violent reaction!
Overall, the illustration shows that there is a great deal in God's way of life that is upsetting, especially when it comes in contact with our worldly way of life before conversion.
Then the Angel tells John, "You have to speak about this." The result of what has taken place is that now he has the inspiration, the information, and the strength to prophesy, or to preach, again. Even though it caused this great queasiness, this upset, the unsettling, sickening, painful feeling, it still filled him and gave him the strength and energy to do the work that he was being given.