The ordinary Greek letter, on papyrus
Greco-Roman, surviving in thousands of examples from Egypt3rd century BC to 3rd century ADThe parallel
A Greek letter ran to a fixed shape: sender, recipient, a greeting, a wish for health or a word of thanks about the reader, then the body, then closing greetings. Paul follows the form in every other letter he wrote, and he follows it here as far as the greeting and then stops. Instead of thanks the Galatians get 'I am amazed how quickly you are deserting the One who called you' (1:6).
Read honestly
A real and pointed contrast, and the readers would have felt it in the first seconds. The missing thanksgiving is the loudest thing on the page, and it tells us how Paul rated the emergency before he had made a single argument. He also breaks the form at the other end, taking the pen himself at 6:11 to write the closing paragraph in his own large hand.

