ChristianAnswers.Net WebBible Encyclopedia
“Letter” in Rom. 2:27, 29 means the outward form.
The “oldness of the letter” (7:6) 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 2 Cor. 3:6, “the letter” means the Mosaic law as a written law.