2:7

“Allah has set a seal on their hearts and on their hearing, (i.e., they are closed from accepting Allah’s Guidance), and on their eyes is a covering. Theirs will be a great torment.”

50:33, 50:37

“Who feared the Most Merciful unseen and came with a heart returning [in repentance].”
“Verily, therein is indeed a reminder for him who has a heart or gives ear while he is heedful.”

40:56

“Indeed, those who dispute concerning the signs of Allah without [any] authority having come to them – there is not within their breasts except pride, [the extent of] which they cannot reach. So seek refuge in Allah . Indeed, it is He who is the Hearing, the Seeing.”

Enjoin Righteousness or Perish

“And fear the Fitnah (affliction and trial) which affects not in particular (only) those of you who do wrong (but it may afflict all the good and the bad people), and know that Allah is Severe in punishment.”

40:35

“Those who dispute about the Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allah, without any authority that has come to them, it is greatly hateful and disgusting to Allah and to those who believe. Thus does Allah seal up the heart of every arrogant, tyrant. (So they cannot guide themselves to the right path).”

37:84

“When he came to his Lord with a pure heart [attached to Allah Alone and none else, worshiping not Allah Alone–true Islamic Monotheism, pure from the filth of polytheism].”

The Hard Heart

Al-hamdu lillahi nahmadhu wa nasta`inuhu wa nastaghfiruhu, wa na`udhu billahi min shururi anfusina wa min sayi’ati a`malina, man yahdihillahu fala mudilla lahu, wa man yudlil fala hadiya lahu. Wa ashadu an la ilaha illallahu wahduhu la sharika lahu, wa anna Muhammadan `abduhu wa rasuluhu.  (Praise is to Allah, we praise Him and we seek His

57:16

“Has not the time come for the hearts of those who believe (in the Oneness of Allah – Islamic Monotheism) to be affected by Allah’s Reminder (this Qur’an), and that which has been revealed of the truth, lest they become as those who received the Scripture [the Taurat (Torah) and the Injeel (Gospel)] before (i.e. Jews and Christians) and the term was prolonged for them and so their hearts were hardened? And many of them were Fasiqun (the rebellious, the disobedient to Allah).”

Tawwakul

Do people think that they will be left alone because they say: “We believe, and will not be tested?

26:89 pt. 2

“Verily, Allah forgives not that partners should be set up with Him (in worship), but He forgives except that (anything else) to whom He wills; and whoever sets up partners with Allah (in worship), he has indeed invented a tremendous sin.” (4:48, 116)