In part 3, Piper looks at several passages of scripture but one in particular that I've never connected to fasting is Matthew 9:14-17, the parable of the new and old wine skins.
Piper's point? Well, you've got to listen to it (or read you can read it). I could just copy and paste the key point, but then you'd miss out on how he unpacks scripture and how it applies to your life and relationship with God.
The teaching-learning process will be most effective when both students and teacher are adequately prepared. -The Law of Readiness, Dr. Howard Hendricks from Teaching to Change LivesFor you to be prepared for the point, as Piper is prepared to give it, I'd encourage you to listen to all 3 parts.