'Letter' in the Bible

And he wrote a letter after this form:

and they, when they came to Caesarea and delivered the letter to the governor, presented Paul also before him.

and shall not the uncircumcision which is by nature, if it fulfil the law, judge thee, who with the letter and circumcision art a transgressor of the law?

but he is a Jew who is one inwardly; and circumcision is that of the heart, in the spirit not in the letter; whose praise is not of men, but of God.

But now we have been discharged from the law, having died to that wherein we were held; so that we serve in newness of the spirit, and not in oldness of the letter.

who also made us sufficient as ministers of a new covenant; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life.