We know from experience that food commonly remains in the mouth for a short period of time, while it remains much longer in the stomach. John, then, will experience short‑term pleasure from eating this scroll, and then (relatively) long‑term discomfort. The fact that this scroll would taste pleasant indicates that its words would be initially pleasing to the hearer, while the fact that this scroll would leave the eater with an upset stomach indicates that the experiences following these (sweet) words would be displeasing.
9 So I went to the angel and told him to give me the little scroll. And he said to me, “Take and eat it; it will make your stomach bitter, but in your mouth it will be sweet as honey.”