In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

And thus We have sent to you (O Muhammad) Ruhan (an Inspiration, and a Mercy) of Our Command. You knew not what is the Book, nor what is Faith? But We have made it (this Qur’an) a light wherewith We guide whosoever of Our slaves We will. And verily, you (O Muhammad) are indeed guiding (mankind) to the Straight Path (i.e. Allah’s religion of Islamic Monotheism). The Path of Allah, to Whom belongs all that is in the heavens and all that is in the earth. Verily, all the matters at the end go to Allah (for decision). [Surah Ash-Shooraa’ Aayaat 52-53]

Shaikh Saaleh Aala Ash-Shaikh [may Allaah preserve him] said: Not every Haafidh of the Qur’aan has this light, rather the one who understands the Qur’aan and is guided by it – the one who adheres to its ordainments, declares lawful what the Qur’aan declares lawful and declares unlawful what the Qur’aan declares unlawful – has light in his heart and clear-sightedness based on how much light he has.

[An Excerpt from ‘Sharh Muqaddimah Usool At-Tafseel Li-Shaikh Al-Islaam Ibn Taymiyyah’ pages 13-14. By Shaikh Saaleh Aala Ash-Shaikh. slightly paraphrased]