The study of the passage should lead to the passage idea. This is a single sentence summary of the passage. Or to put it another way, it is the passage distilled into a single sentence. There are several ways to mis-distill a passage. For instance:
1. Misdistillation by searching for the best verse. This is a relatively elementary error, but not too unusual. The passage is read and the preacher decides, “Verse 7, that’s the one, I’ll make verse 7 the passage idea!” Now there are occasions where a particular verse, or phrase, or sentence, may function as a passage idea. But typically this is not the case. The goal is to summarize the whole text, not just pick out a part that stands out to you. You should be able to test the passage idea against the rest of the passage and find that it is all feeding into the idea.
2. Misdistillation by scouting for commands only. This is a common mistake, driven by theology as much as anything. A theology that says people need to be informed and exhorted will probably be looking for the imperatives in the passage. Again, the passage idea may well tie in to an exhortation in a passage, but it is to be the summary of the whole, not just an imperatival mood filter. Be sensitive to what the passage is trying to do in the context of the whole book. This may not be a commanding passage at all. Take off the coloured glasses and try to see the passage on its own terms.
3. Misdistillation by spotting a meaty doctrinal truth. This is a tempting error. You scan the passage and notice a reference to a truth you’d love to expand at length. Voila. Main idea! But that idea may only be part of the whole, or even a minor player in the choreographed presentation of all the players. For instance, sometimes Paul makes a theologically meaty reference in the introduction to a prayer. Be sure to study the whole and distill the whole, don’t just get excited because there is a passing reference to sovereignty, or whatever.
When you are wrestling with a passage, be sure to distill the whole passage down into the passage idea. Any other approach and you won’t be preaching the whole passage.