It is a remarkable sermon because Piper (for good reason) doesn't believe the story to be part of the inspired record of John's gospel. However, he does believe that the story is true. In other words, the story is probably historically accurate, but it shouldn't be included in the book of John.
In the first part of the sermon, he explains the discipline of textual criticism. In the second part he draws out the lessons from a story that is not actually inspired.
Remarkably, the sermon is both informative and inspiring even though the text isn't inspired!
Here are a few important quotes:
Now the question is: What should I, the preacher, do with this story? Both Don Carson and Bruce Metzger think the story probably happened. In other words, they think this is a real event from Jesus's life, and the story circulated and later was put in the Gospel of John. Metzger says, "The account has all the earmarks of historical veracity" (Textual Commentary, p. 220). And Carson says, "There is little reason for doubting that the event here described occurred" (The Gospel According to John, p. 333).The most remarkable point of this story is that Jesus exalts himself above the Law of Moses, changes its appointed punishment, and reestablishes righteousness on the foundation of grace. I don't doubt that this is why the story was preserved. It is an amazing storyThe story may not belong to John's Gospel. In fact, the story may never have happened. But this point of the story is unshakably true. This is the pervasive message of the New Testament. Jesus exalted himself above the Law. He wrote it! Jesus altered some of its sanctions. He pointed to its main goal of Christ-exalting love. And he reestablished righteousness on the basis of an experience of grace.