The Quran

أَلا إِنَّهُم يَثنونَ صُدورَهُم لِيَستَخفوا مِنهُ ۚ أَلا حينَ يَستَغشونَ ثِيابَهُم يَعلَمُ ما يُسِرّونَ وَما يُعلِنونَ ۚ إِنَّهُ عَليمٌ بِذاتِ الصُّدورِ

Muhammad Asad

Oh, verily, they [who are bent on denying the truth of this divine writ] are enshrouding their hearts in order to hide from Him.¹ Oh, verily, [even] when they cover themselves with their garments [in order not to see or hear],² He knows all that they keep secret as well as all that they bring into the open -for, behold, He has full knowledge of what is in the hearts [of men].

Since the people referred to in this verse obviously do not believe in the divine origin of Muhammad's message, their "hiding from God" can have, in this context, only one meaning namely, that of a metaphor for their unwillingness to listen to the truth which emanates from Him: and this also explains the statement that they are "enshrouding their hearts" (lit., "bosoms", as at the end of this verse), i.e., are allowing their hearts and minds to remain wrapped-up in prejudices, thus making them impervious to spiritual perception. See in this connection 8:55 and the corresponding note 58.
The above interpolation corresponds to the meaning given to the preceding phrase by most of the lexicographers (cf. Lane VI, 2262).