Nun is one of the letters of the Arabic alphabet; in the same way, various other letters introduce some surahs (chapters) of the Quran, while no commentator-nor even the Prophet himself-is able to say exactly what they mean or what their presence at the beginning of a chapter symbolizes. Thus, at the very moment when the Creator swears “by the pen” and confirms the necessity of knowledge conveyed to human beings, He opens the verses with a mysterious letter, nun, expressing the limits of human knowledge. The dignity of humankind, conferred by knowledge, cannot be devoid of the humility of reason aware of its own limits and thereby recognizing the necessity of faith. Accepting, and accepting not to understand, the mysterious presence of the letter nun requires faith; understanding and accepting the unmysterious statements of the verses that follow require the use of a reason that is active but necessarily-and indeed naturally–humbled.
From Tariq Ramadan – In the Footsteps of the Prophet