Second Corinthians 3:6 says, “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.” With these words, Paul summarizes the key difference between the Old and New Testaments: the first covenant was based on obedience to the written law (the “letter”), but the second covenant is based on the blood of Christ and sealed by the Holy Spirit.
There are two parts to this answer, as we look at both the letter and the Spirit.
First, what does Paul mean by “the letter kills”? Simply that the Old Testament Law, which is good and perfect (Psalm 19:7), reveals all people as law-breakers (Galatians 3:10). The law “kills” in that the penalty for breaking God’s law is eternal death in hell (Romans 6:23; Revelation 21:8). As God told Moses the lawgiver, “Whoever has sinned against me I will blot out of my book” (Exodus 32:33). Even if you sin only once in your whole life, it’s the same as breaking all of God’s laws (James 2:10), just as breaking only one link in a chain breaks the whole chain.
The written law—“the letter”—was chiseled in stone by the finger of God and is the unchanging standard by which all are judged. The law cannot give us righteousness or eternal life in heaven (Galatians 2:16). It can only condemn us as sinners, and the sentence is death. Heaven is where perfection is required (Matthew 5:20, 48; 19:16–21), and “the law made nothing perfect” (Hebrews 7:19).
Second, what does Paul mean by “the Spirit gives life”? Simply that the Holy Spirit rescues us from our hopeless situation. God saves us from death and grants us eternal life when we are born again through the power of the Holy Spirit. Jesus said, “Flesh gives birth to flesh, but the Spirit gives birth to spirit” (John 3:6), and, later, “The Spirit gives life; the flesh counts for nothing. The words I have spoken to you—they are Spirit and they are life” (John 6:63).
The Holy Spirit was active in the Incarnation of our Savior (Luke 1:35). It was through the Holy Spirit that Jesus offered Himself as a sacrifice to God for our sins (Hebrews 9:14). The Spirit is the cause of the new birth (John 3:3–8). It is the Spirit who lives in believers (John 14:17), seals them (Ephesians 1:13), and sanctifies them (Romans 15:16).
Jesus came to give us an abundant life, or life “to the full” (John 10:10). The Holy Spirit living in believers is how Jesus fulfills that promise. The abundant Christian life is marked by the fruit of the Spirit, which is “love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control” (Galatians 5:22–23). The Old Testament Law could not produce any of that fruit; only the Holy Spirit can, as He lives in us.
The Spirit gives life in that He enables us to reach God’s ultimate goal for us, to be transformed into the glorious image of God’s own Son (2 Corinthians 3:18; also see Romans 8:28–30). Until the day that we see Christ, the Spirit intercedes with God on our behalf, ensuring our continued forgiveness and preserving the promise of God (Romans 8:26–27).
“The letter kills, but the Spirit gives life” (2 Corinthians 3:6). Elsewhere, Paul teaches the same truth: “But now, by dying to what once bound us, we have been released from the law so that we serve in the new way of the Spirit, and not in the old way of the written code” (Romans 7:6).