'Letter' in the Bible

A man who is physically uncircumcised, but who fulfills the law, will judge you who are a lawbreaker in spite of having the letter of the law and circumcision.

On the contrary, a person is a Jew who is one inwardly, and circumcision is of the heart—by the Spirit, not the letter. That man’s praise is not from men but from God.

But now we have been released from the law, since we have died to what held us, so that we may serve in the new way of the Spirit and not in the old letter of the law.