in Ro 2:27,29 means the outward form. The "oldness of the letter" (Ro 2:6) is a phrase which denotes the old way of literal outward obedience to the law as a system of mere external rules of conduct. In 2Co 3:6, "the letter" means the Mosaic law as a written law. (See Writing.)
who will render to every one according to his works;
Yea, those who are by nature uncircumcised, if they perform the law, will judge thee, who having a written Law and circumcision, art a breaker of the Law.
but he is a Jew who is one inwardly; and circumcision is of the heart, spiritual, not literal, whose praise is not of men, but of God.
who also gave us ability to be ministers of a new covenant, not of the letter, but of the Spirit; for the letter killeth, but the Spirit giveth life.