If the Prophet forged anything against Allah, then Allah would punish Him
Allah says,
وَلَوْ تَقَوَّلَ عَلَيْنَا
(And if he had forged a false saying concerning Us,) meaning, `if Muhammad forged something against Us, as they claim, and added or removed anything from the Message, or said anything from himself while attributing it to Us, then We would surely be swift in punishing him. And of course, Muhammad ﷺ did not do any of this (as the disbelievers claimed).' Thus, Allah says,
لأَخَذْنَا مِنْهُ بِالْيَمِينِ
(We surely would have seized him by his right hand,) It has been said that this means, `We would seize him by the right hand because it is more stronger in grabbing.'
ثُمَّ لَقَطَعْنَا مِنْهُ الْوَتِينَ
(And then We certainly would have cut off Al-Watin from him,) Ibn `Abbas said, "It (Al-Watin) refers to the artery of the heart, and it is the vein that is attached to the heart." This has also been said by `Ikrimah, Sa`id bin Jubayr, Al-Hakim, Qatadah, Ad-Dahhak, Muslim Al-Batin and Abu Sakhr Humayd bin Ziyad. Muhammad bin Ka`b said, "It (Al-Watin) is the heart, its blood, and whatever is near it." Concerning Allah's statement,
فَمَا مِنكُم مِّنْ أَحَدٍ عَنْهُ حَـجِزِينَ
(And none of you could have prevented it from him.) means, `none of you would be able to come between Us and him if We wanted to do any of this to him.' The meaning behind all of this is to say that he (Muhammad ﷺ) is truthful, righteous and guided because Allah determined what he is to convey from Him, and Allah helps him with fantastic miracles and definite proofs. Then Allah says,
وَإِنَّهُ لَتَذْكِرَةٌ لِّلْمُتَّقِينَ
(And verily, this (Qur'an) is a Reminder for those who have Taqwa.) meaning, the Qur'an. This is just as Allah says,
قُلْ هُوَ لِلَّذِينَ ءَامَنُواْ هُدًى وَشِفَآءٌ وَالَّذِينَ لاَ يُؤْمِنُونَ فِى ءَاذَانِهِمْ وَقْرٌ وَهُوَ عَلَيْهِمْ عَمًى
(Say: "It is for those who believe, a guide and a healing. And as for those who disbelieve, there is heaviness (deafness) in their ears, and it (the Qur'an) is blindness for them.") Then Allah says,
وَإِنَّا لَنَعْلَمُ أَنَّ مِنكُم مُّكَذِّبِينَ
(And verily, We know that there are some among you that deny (this Qur'an). ) meaning, with this explanation and clarification, there will still be among you those who reject the Qur'an. Then Allah says,
وَإِنَّهُ لَحَسْرَةٌ عَلَى الْكَـفِرِينَ
(And indeed it (this Qur'an) will be an anguish for the disbelievers (on the Day of Resurrection).) Ibn Jarir said, "And verily this rejection will be anguish for the disbelievers on the Day of Judgement." He (Ibn Jarir) also mentioned a similar statement from Qatadah. It is possible that the meaning of the pronoun (it) may also refer to the Qur'an, in which case the verse would mean that the Qur'an and belief in it are a cause of anguish for the disbelievers. This is as Allah says,
كَذَلِكَ سَلَكْنَاهُ فِي قُلُوبِ الْمُجْرِمِينَ لاَ يُؤْمِنُونَ بِهِ
(Thus have We caused it (the denial of the Qur'an) to enter the hearts of the criminals. They will not believe in it.) (26:200,201) And Allah said,
وَحِيلَ بَيْنَهُمْ وَبَيْنَ مَا يَشْتَهُونَ
(And a barrier will be set between them and that which they desire) (34:54). Therefore, Allah says here,
وَإِنَّهُ لَحَقُّ الْيَقِينِ
(And verily, it (this Qur'an) is an absolute truth with certainty.) meaning, the right and truthful news in which there is no doubt, suspicion or confusion. Then Allah says,
فَسَبِّحْ بِاسْمِ رَبِّكَ الْعَظِيمِ
(So glorify the Name of your Lord, the Most Great.) meaning, He Who sent down this magnificent Qur'an. This is the end of the explanation (Tafsir) of Surat Al-Haqqah. And to Allah belong all praise and blessings.