8/6 2 Corinthians 3:3

3 And you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.

Paul is referring to a prophecy given by Ezekiel (Ezekiel 36:26) and combining it with one from Jeremiah (Jeremiah 31:31). The days would come when God would take the heart of stone from Israel and give them a heart of flesh. God would make a New Covenant with them, writing the law on their hearts. Paul is saying that day has come and their transformed hearts are proof of it.

The apostles explained the New Covenant, salvation by faith in the work of Jesus. The Holy Spirit is the law within their hearts. Instead of the historical stony heart of Israel, the believers in Jesus have a heart of flesh, a gift from God, to desire to do the will of God and to obey rather than to rebel. By the grace of God they are done with idolatry. In the language of the prophets, Paul was saying that the kingdom of God had come.

That was Paul's proof of being an apostle. He had helped them enter the Kingdom. He quickly goes on to add that it wasn't because he is so great, but that God has equipped the apostles with everything needed to usher in the Kingdom (2 Corinthians 3:5-6). The glory of this New Covenant far surpasses the old. The old came with the shaking of Sinai, thunder, and a fearful warning of what it meant to disobey. How much more glorious is the new that has come with the promise of eternal life in Jesus!

Consider: Is your life a demonstration of the letter written on your heart of flesh? Are you living daily in the glorious New Covenant?