The letter kills, but the Spirit gives life. (2 Cor. 3:6)
The law of God is holy and just and good; it is inexorable, and we have fallen under its just condemnation. That is at the bottom of what Paul means by the “the letter kills.’ He does not mean that attention to pedantic details shrivels and deadens the soul. No doubt that is true, within certain spheres; it is a useful thought. But it is trivial indeed compared with what Paul means. Something far more majestic, far more terrible, is meant by the Pauline phrase. The letter that Paul means is the dreadful handwriting of ordinances that was against us, and the death with which it kills is the eternal death of those who are forever separated from God.
But that is not all of the text. The letter kills, Paul says, but the Spirit makes alive. There is no doubt about what he means by ‘the Spirit.’ He does not mean that spirit of the law as contrasted with the letter; he certainly does not mean the lax interpretation of God’s commands which is dictated by human lust or pride; he certainly does not mean the spirit of man. No real student of Paul has doubted, so far as I know, but that he means the Spirit of God. God’s law brings eternal death because of sin; but God’s Spirit, shed abroad in the heart, brings life. The thing that is written kills, but the Holy Spirit in the heart gives life.
The contrast runs all through the New Testament. Hopelessness under the law is described in the seventh chapter of Romans. “Oh wretched man that I am! Who shall deliver me from the body of this death?” But this hopelessness is transcended by the gospel. “I thank God through Jesus Christ our Lord… For the law of the Spirit of life in Jesus Christ hath made me free from the law of sin and death.’ The law’s sentence of condemnation was borne for us by Christ who suffered in our stead; The handwriting of ordinances which was against us—the dreadful ‘letter’ of which Paul speaks in our text—was nailed to the cross.
-J. Gresham Machen