I suppose there are several easy mistakes to make when it comes to getting the main idea of a passage. I’d like to point out one today.
Do not look for the biggest detail of the passage and then omit the rest of the passage. It may be tempting to look for the weightiest element in a passage and make that the main idea. Equally, it may be a misunderstanding of the process to search for the biggest point and then miss the rest.
What we should be doing is distilling the whole passage, allowing every detail (big or small), to influence the statement of the main idea. Some details may not be visible in the wording of the main idea. Perhaps they influence the tone or the feel of the idea. Some details are developments of the main idea (perhaps explaining, or proving, or applying it) and consequently may not show in the statement. However, it is important to approach getting the main idea in the right way:
The right way: every detail feeds into our understanding of the whole, which is then summarized or distilled into one sentence.
The wrong way: only the most significant detail (or even the most attractive or preachable detail) is used to define the main idea, all other details are skipped or omitted.