...the letter kills but the Spirit gives life.

Notes from Calvin on 2 Corinthians 3:6

There is, however, no doubt, that by the term letter, he means the Old Testament, as by the term spirit he means the gospel; for, after having called himself a minister of the New Testament, he immediately adds, by way of exposition, that he is a minister of the spirit, and contrasts the letter with the spirit.

We must now enquire into the reason of this designation. The exposition contrived by Origen has got into general circulation — that by the letter we ought to understand the grammatical and genuine meaning of Scripture, or the literal sense, (as they call it,) and that by the spirit is meant the allegorical meaning, which is commonly reckoned to be the spiritual meaning. Accordingly, during several centuries, nothing was more commonly said, or more generally received, than this — that Paul here furnishes us with a key for expounding Scripture by allegories, while nothing is farther from his intention.

For by the term letter he means outward preaching, of such a kind as does not reach the heart; and, on the other hand, by spirit he means living doctrine, of such a nature as worketh effectually (1 Thess 2:12) on the minds of men, through the grace of the Spirit. By the term letter, therefore, is meant literal preaching — that is, dead and ineffectual, perceived only by the ear. By the term spirit, on the other hand, is meant spiritual doctrine, that is, what is not merely uttered with the mouth, but effectually makes its way to the souls of men with a lively feeling. For Paul had an eye to the passage in Jeremiah, that I quoted a little ago, (Jer 31:31) where the Lord says, that his law had been proclaimed merely with the mouth, and that it had, therefore, been of short duration, because the people did not embrace it in their heart, and he promises the Spirit of regeneration under the reign of Christ, to write his gospel, that is, the new covenant, upon their hearts. Paul now makes it his boast, that the accomplishment of that prophecy is to be seen in his preaching, that the Corinthians may perceive, how worthless is the loquacity of those vain boasters, who make incessant noise while devoid of the efficacy of the Spirit.

It is asked, however, whether God, under the Old Testament, merely sounded forth in the way of an external voice, and did not also speak inwardly to the hearts of the pious by his Spirit.

I answer in the first place, that Paul here takes into view what belonged peculiarly to the law; for although God then wrought by his Spirit, yet that did not take its rise from the ministry of Moses, but from the grace of Christ, as it is said in John 1:17 —
The law was given by Moses;but grace and truth came by Jesus Christ.

True, indeed, the grace of God did not, during all that time, lie dormant, but it is enough that it was not a benefit that belonged to the law. For Moses had discharged his office, when he had delivered to the people the doctrine of life, adding threatenings and promises. For this reason he gives to the law the name of the letter, because it is in itself a dead preaching; but the gospel he calls spirit, because the ministry of the gospel is living, nay, lifegiving.

I answer secondly, that these things are not affirmed absolutely in reference either to the law or to the gospel, but in respect of the contrast between the one and the other; for even the gospel is not always spirit. When, however, we come to compare the two, it is truly and properly affirmed, that the nature of the law is to teach men literally, in such a way that it does not reach farther than the ear; and that, on the other hand, the nature of the gospel is to teach spiritually, because it is the instrument of Christ’s grace. This depends on the appointment of God, who has seen it meet to manifest the efficacy of his Spirit more clearly in the gospel than in the law, for it is his work exclusively to teach effectually the minds of men.