Quran verse on the relationship between belief in Allah and attaining understanding
Who are men of understanding?
Those who listen to the Word [good advice La ilaha illallah - (none has the right to be worshipped but Allah) and Islamic Monotheism] and follow the best thereof (i.e. worship Allah Alone, repent to Him and avoid Taghut) those are (the ones) whom Allah has guided and those are men of understanding.
Quran (Surah Az-Zumr, Verse 18)
