Moses comes to the forefront of Paul’s thoughts in chapter 3. He’s already present in 2:14-17 to a limited degree in the triumphal procession and then to a greater measure in the aroma of life and death (Numbers 16). Still, both of those are more subtle than chapter 3 where he rises above the water line and is not only named by Paul but incorporated into important comparisons/contrasts with him and his ministry.
Apparently, the Corinthians (or the “super-apostles”) had brought up the issue of letters of recommendation (v. 1). Rather than being offended that they were asking an apostle of Jesus for such a credential, Paul used the occasion to teach them some important things. (I’m sure there’s a lesson for us there somewhere).
He tells the Corinthians that such documentation is unnecessary because “you are a letter from Christ, the result of our ministry, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts” (v. 3). He doesn’t need to provide them with outside proof because they themselves are exhibit A of his ministry.
Furthermore, the Corinthians were overvaluing such letters. As McGuiggan points out, the problem with them was that they were just words on a piece of paper that meant nothing if the truth of them wasn’t internalized by the people who received them. You can take a glowing letter of reference to your interview, but if the person conducting it pays no attention to the document, then it is just words on a piece of paper. In such a case though, the letter becomes their judge and speaks condemnation to them when you are hired by a rival company and your interviewer’s boss want to know why you weren’t hired by them.
Moses was a minister of a glorious covenant who had two letters (tablets) from God no less, yet he was rejected by the bulk of Israel. Thus, the covenant he gave became a “ministry that brought death” (v. 7) and “condemnation” (v. 9). It did this not because it was incapable of bringing life (see Deuteronomy 30:11-20), but because it remained just a “letter”—something external to the bulk of the nation. This is why when Paul says, “The letter kills” (v. 6)—it is not a reference to the medium of the message (both Old and New Testaments were ultimately conveyed in written form), but to the reception of it.
Paul carried the Corinthians around in his heart (v. 2). His ministry was not of letters to be received externally (as the Israel had done) but of allowing the Spirit the write on your heart (as the Corinthians did). Now they wanted to go back to external letters and it was a big step backwards!