Charles Hodge on 2 Corinthians 3:6:

An Exposition of the Second Epistle to the Corinthians. Grand Rapids: Eerdmans, n.d., 56-58 (Language modernized by Nate Milne where necessary).

For the letter (i.e., the law) kills, but the spirit (i.e., the gospel) gives life. This is the reason why God has made Paul the minister of the Spirit. “God had made us able minsters not of the law but of the gospel, for the law kills, but the gospel gives life.” This passage and the following context present two important questions. First, “In what sense does the law kill?” And second, “How is it that the apostle attributes to the Mosaic system this purely legal character, when he elsewhere so plainly teaches that the gospel was witnessed or taught both in the law and the prophets?”

As to the former of these questions, the answer furnished by the Scriptures is plain. The law demands perfect obedience. It says, “Do this and live” (Rom. 10:5; Gal. 3:12), and “Cursed is everyone who does not continue in all things written in the book of the law to do them” (Gal. 3:10). As no man renders this perfect obedience, the law condemns him. It pronounces on him the sentence of death. This is one way in which it kills.

In the second place, it produces the knowledge or consciousness of sin, and of course of guilt, that is, of just exposure to the wrath of God. Thus again it slays. And thirdly, by presenting the perfect standard of duty, which cannot be seen without awakening the sense of obligation to be conformed to it, while it imparts no disposition or power to obey, it exasperates the soul and thus again it brings forth fruit unto death. All these effects of the law are systematically presented by the apostle in Romans 6 & 7, and Galatians 3.

The second question is more difficult. Every reader of the New Testament must be struck with the fact that the apostle often speaks of the Mosaic law as he does of the moral law considered as a covenant of works; this is, presenting the promise of life on the condition of perfect obedience. He represents it saying, “Do this and live;” as requiring works, and not faith, as the condition of acceptance (Rom. 10:5-10; Gal. 3:10-12). He calls it a ministration of death and condemnation. He denies that it can give life (Gal. 3:21). He tells those who are of the law (that is, Judaizers) that they had fallen from grace; that is, had renounced the gratuitous method of salvation, and that Christ should profit them nothing (Gal. 5:2, 4).

In short, when he uses the word law, and says that by the law is the knowledge of sin, that it can only condemn, that by its works no flesh can be justified, he includes the Mosaic law; and in the epistle to the Galatians all these things are said with special reference to the law of Moses.

On the other hand, however, he teaches that the plan of salvation has been the same from the beginning; that Christ was the propitiation for the sins committed under the old covenant; that men were saved then as now by faith in Christ; that this mode of salvation was revealed to Abraham and understood by him, and taught by Moses and the prophets. This view is presented repeatedly in Paul’s epistles, and is argued out in due form in Rom. 3:21-31; Rom. 4; & Gal. 3.

To reconcile these apparently conflicting representations it must be remembered that the Mosaic economy was designed to accomplish different objects, and is therefore presented in Scripture under different aspects. What, therefore, is true of it under one aspect, is not true under another.

1. The law of Moses was, in the first place, a re-enactment of the covenant of works. A covenant is simply a promise suspended upon a condition. The covenant of works, therefore, is nothing more than the promise of life suspended on the condition of perfect obedience. The phrase is used as a concise and convenient expression of the eternal principles of justice on which God deals with rational creatures, and which underlie all dispensations, the Adamic, Abrahamic, Mosaic, and Christian. Our Lord said to the lawyer who asked what he should do to inherit eternal life, “‘What is written in the law? What do you read?’ And he, answering, said, ‘You shall love the Lord your God with all your heart, and with all your soul, and with all your strength, and with all your mind; and your neighbor as yourself.’ And he said unto him, ‘You have answered rightly, do this and you shall live’” (Luke 10:26-28). This is the covenant of works. It is an immutable principle that where there is no sin there is no condemnation, and where there is sin there is death. This is all that those who reject the gospel have to fall back upon. It is this principle which is rendered so prominent in the Mosaic economy as to give it is character of law. Viewed under this aspect is is the ministration of condemnation and death.

2. The Mosaic economy was also a national covenant; that is, it presented national promises on the condition of national obedience. Under this aspect also it was purely legal.

3. But, as the gospel contains a renewed revelation of the law, so the law of Moses contained a revelation of the gospel. It presented in its priesthood and sacrifices, as types of the office and work of Christ, the gratuitous method of salvation through a Redeemer. This necessarily supposes that faith and not works was the condition of salvation. It was those who trusted, not those free from sin, who were saved. Thus Moses wrote of Christ (John 5:46); and thus the law and the prophets witnessed of a righteousness of faith (Rom. 3:21). When therefore the apostle spoke of the old covenant under its legal aspect, and especially when speaking to those who rejected the gospel and clung to the law of Moses as law, then he says, it kills, or is the ministration of condemnation. But when viewing it, and especially when speaking of those who viewed it as setting forth the great doctrine of redemption through the blood of Christ, the represented it as teaching his own doctrine.

The law, in every form, moral or Mosaic, natural or revealed, kills. In demanding works as the condition of salvation, it must condemn all sinners. But the gospel, whether as revealed in the promise to Adam after his fall, or in the promise to Abraham, or in the writings of Moses, or in its full clearness in the New Testament, gives life. As the old covenant revealed both the law and the gospel, it either killed or gave life, according to the light in which it was viewed. And therefore Paul sometimes says it does the one, and sometimes the other.

But the spirit gives life. The spirit, or the gospel, gives life in a sense correlating to that in which the letter (i.e., the law) kills.

1. By revealing a righteousness adequate to our justification, and thus delivering us from the sentence of death.

2. By producing the assurance of God’s love and the hope of his glory in the place of a dread of his wrath.

3. By becoming, through the agency of the Holy Spirit, an inward principle or power transforming us into the image of God; instead of a mere outward command.