Allah the Exalted said: << وربك يعلم ما تكن صدورهم وما يعلنون >

The address in this verse and the one before it is either to the Messenger (peace and blessings be upon him), or to everyone to whom it is appropriate to direct the address, their hearts conceal from disbelief and other things meaning they keep secret and hide that is, their hearts. The reason the chest is mentioned is that the heart is in it, and the heart is connected to what? To the chest. Therefore, the chest is the place that shelters the heart and covers it. Also, whatever is hidden in the heart, Allah the Exalted knows it. And the author's statement:"from disbelief and other things" is correct; from disbelief and other than disbelief, nothing is hidden from Him of what is in the heart. Allah the Exalted said: [Q:16] it whispers to it, meaning it informs it. So, Glorified and Exalted is He, nothing is hidden from Him; rather, He knows what you do not know. For example, He knows that you will whisper such and such on such and such a day before it happens from you, is that not so? Yes. with their tongues of that" they declare, they make manifest. The author's restriction of making manifest to the tongues is deficient, because declaration may be by the tongue or by other limbs. It may be by the tongue, so one speaks, or by other limbs, so one acts with his hands or feet or eyes or otherwise. Thus, it is broader than what the author said.