So let’s say you are preaching on Ephesians 2:1-10. And you happen to see on Facebook that the Apostle Paul is preaching at an event not far from you on that very text, just two days before you are due to preach. Let’s assume he is not able to come and take your preaching engagement, but you can get to hear his.
After he preaches the passage, explaining his way through it, you decide to cut to the bottom line. You approach him afterwards and get to him before any of the others who line up behind you. “Thanks Paul, great to meet you, so you are saying, in Ephesians 2:1-10…” then you just decide to state your main idea of the passage to him, “that God saved us by grace, making us alive so that we can do good works?”
If Paul’s response might be, “uh, yes, sort of, but what I’m mostly saying is that it is all of God’s grace that he has made us who were dead, alive with Christ . . .” then you should change your message. If your main idea is not what he’d say his main idea was in the passage, then your main idea should change.
Remember, as a preacher your task is not to come up with your own message somehow based on a text. Your job is to re-present the message of that text, targeted to a new audience and situation, but remaining genuinely faithful to the intent of the author. Be nice to ask him in person, but let’s be sure to check our main idea against the text itself, and to do so more than once. Feel free to ask someone else too, not the author, but someone who will look at the text carefully and test your idea.