The Quran

وَيَعبُدونَ مِن دونِ اللَّهِ ما لَم يُنَزِّل بِهِ سُلطانًا وَما لَيسَ لَهُم بِهِ عِلمٌ ۗ وَما لِلظّالِمينَ مِن نَصيرٍ

Muhammad Asad

And yet,¹ they [who claim to believe in Him often] worship [other beings or forces] beside God -something for which He has never bestowed any warrant from on high,² and [of the reality] whereof they cannot have any knowledge:³ and such evildoers shall have none to succour them [on Judgment Day].

I.e., despite their awareness that God alone knows all and is, therefore, unique in His all-embracing Presence.
See surah 3, note 106.
I.e., through independent reasoning or observation.