'Letter' in the Bible

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.