“He has made us competent as ministers of a new covenant–not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.” (2 Corinthians 3:6 NIV84)
Paul is so adamant that we make a proper distinction between law and Gospel. Throughout his ministry he had the Judaisers (Jewish Christians who wanted to add the requirement of the Mosaic law to the Gospel of Jesus Christ) constantly undermining his evangelistic work. Paul understood this one point very well: If you add a single law to the gospel, the gospel loses its freedom and is no longer “gospel” (good news). If we were the thief on the cross, and because of our faith in Jesus He said to us that we would be in Paradise with Him that very day, that is certainly good news. But if Jesus said to us, “Hold on! One more thing!”, and that thing was a requirement we had to fulfill, we would have a problem. How would we know if we did our “one thing” sufficiently, purely, adequately, perfectly enough. The requirement of the law always leaves us wanting. The looming question will always be, “Have we done our part well enough to please Him?”
The letter of the law kills. It shows us that we have fallen short of God’s requirement, no matter what the request. Only the gospel tells us that Jesus sufficiently fulfilled the law in our place and bore the consequences of our sins so completely that by faith in Him alone, we have eternal life. Now note, the Spirit of God works through both the law and the gospel. He works through the law to show us our sin so we will move to our Savior. The Spirit works through the gospel to show us the free (no strings attached) salvation we have in Jesus Christ. We must resist the temptation to add legal requirements to the Gospel, and thereby, robbing it of its life-giving ability. Always remember, the letter kills, but the Spirit gives life.