Allah's saying;

﴿أَمْ يَقُولُونَ افْتَرَى عَلَى اللَّهِ كَذِباً فَإِن يَشَإِ اللَّهُ يَخْتِمْ عَلَى قَلْبِكَ﴾

(Or say they: "He has invented a lie against Allah'' If Allah willed, He could have sealed up your heart.) means, `if you had invented any lies against Him, as these ignorant people claim,'

﴿يَخْتِمْ عَلَى قَلْبِكَ﴾

(He could have sealed up your heart.) means, `and thus caused you to forget what had already come to you of the Qur'an.' This is like the Ayah:

﴿وَلَوْ تَقَوَّلَ عَلَيْنَا بَعْضَ الاٌّقَاوِيلِ - لأَخَذْنَا مِنْهُ بِالْيَمِينِ - ثُمَّ لَقَطَعْنَا مِنْهُ الْوَتِينَ - فَمَا مِنكُم مِّنْ أَحَدٍ عَنْهُ حَـجِزِينَ ﴾

(And if he had forged a false saying concerning Us (Allah), We surely would have seized him by his right hand, and then We certainly would have cut off his life artery, And none of you could have withheld Us from (punishing) him.) (69:44-47) which means, `We would have wrought the utmost vengeance upon him, and no one among mankind would have been able to protect him.' And Allah said:

﴿وَيُحِقُّ الْحَقَّ بِكَلِمَـتِهِ﴾

(and establishes the truth by His Word.) means, He establishes it and strengthens it and makes it clear by His Words, i.e., by His evidence and signs.

﴿إِنَّهُ عَلِيمٌ بِذَاتِ الصُّدُورِ﴾

(Verily, He knows well what are in the breasts.) means, all that is hidden in the hearts of men.

﴿وَهُوَ الَّذِى يَقْبَلُ التَّوْبَةَ عَنْ عِبَادِهِ وَيَعْفُواْ عَنِ السَّيِّئَـتِ وَيَعْلَمُ مَا تَفْعَلُونَ - وَيَسْتَجِيبُ الَّذِينَ ءَامَنُواْ وَعَمِلُواْ الصَّـلِحَـتِ وَيَزِيدُهُم مِّن فَضْلِهِ وَالْكَـفِرُونَ لَهُمْ عَذَابٌ شَدِيدٌ - وَلَوْ بَسَطَ اللَّهُ الرِّزْقَ لِعِبَادِهِ لَبَغَوْاْ فِى الاٌّرْضِ وَلَـكِن يُنَزِّلُ بِقَدَرٍ مَّا يَشَآءُ إِنَّهُ بِعِبَادِهِ خَبِيرُ بَصِيرٌ - وَهُوَ الَّذِى يُنَزِّلُ الْغَيْثَ مِن بَعْدِ مَا قَنَطُواْ وَيَنشُرُ رَحْمَتَهُ وَهُوَ الْوَلِىُّ الْحَمِيدُ ﴾

(25. And He it is Who accepts repentance from His servants, and forgives sins, and He knows what you do.) (26. And He answers those who believe and do righteous good deeds, and gives them increase of His bounty. And as for the disbelievers, theirs will be a severe torment.) (27. And if Allah were to extend the provision for His servants, they would surely rebel in the earth, but He sends down by measure as He wills. Verily, He is, in respect of His servants, the Well-Aware, the All-Seer.) (28. And He it is Who sends down the rain after they have despaired, and spreads His mercy. And He is the Wali, Worthy of all praise.)