قُلْ
(Say) -- `O Muhammad, to these idolators who disbelieve in the Qur'an'--
أَرَءَيْتُمْ إِن كَانَ
(Tell me, if it) -- this Qur'an --
مِنْ عِندِ اللَّهِ ثُمَّ كَفَرْتُمْ بِهِ
(is from Allah, and you disbelieve in it) means, `what do you think your position is before the One Who revealed it to His Messenger' Allah says:
مَنْ أَضَلُّ مِمَّنْ هُوَ فِى شِقَاقٍ بَعِيدٍ
(Who is more astray than one who is in opposition far away.) means, he is in a state of disbelief, stubbornness and opposition to the truth, and is following a path that leads far away from guidance. Then Allah says:
سَنُرِيهِمْ ءَايَـتِنَا فِى الاٌّفَاقِ وَفِى أَنفُسِهِمْ
(We will show them Our signs in the universe, and within themselves,) meaning, `We will show them Our evidence and proof that the Qur'an is true and has indeed been sent down from Allah to the Messenger of Allah ﷺ , through external signs,
فِى الاٌّفَاقِ
(in the universe), ' such as conquests and the advent of Islam over various regions and over all other religions. Mujahid, Al-Hasan and As-Suddi said, "And signs within themselves means, the battle of Badr, the conquest of Makkah, and other events which befell them, where Allah granted victory to Muhammad ﷺ and his Companions, and inflicted a humiliating defeat on falsehood and its followers." It may also be that what is meant here is man and his physical composition. As detailed in the science of anatomy, which indicates the wisdom of the Creator -- may He be blessed and exalted -- and the different inclinations and opposite natures that people have, good and bad, etc., and the (divine) decrees to which man is subjected, having no power to change and over which he has no control.
حَتَّى يَتَبَيَّنَ لَهُمْ أَنَّهُ الْحَقُّ أَوَلَمْ يَكْفِ بِرَبِّكَ أَنَّهُ عَلَى كُلِّ شَىْءٍ شَهِيدٌ
(until it becomes manifest to them that this (the Qur'an) is the truth. Is it not sufficient in regard to your Lord that He is a Witness over all things) means, sufficient is Allah as a Witness over the words and deeds of His servants, and He bears witness that Muhammad ﷺ is telling the truth in what he says, as Allah says:
لَّـكِنِ اللَّهُ يَشْهَدُ بِمَآ أَنزَلَ إِلَيْكَ أَنزَلَهُ بِعِلْمِهِ
(But Allah bears witness to that which He has sent down unto you; He has sent it down with His knowledge) (4:166).
أَلاَ إِنَّهُمْ فِى مِرْيَةٍ مِّن لِّقَآءِ رَبِّهِمْ
(Lo, Verily, they are in doubt concerning the meeting with their Lord.) means, they doubt that the Hour will come, they do not think about it or strive for it or pay heed to it; on the contrary, they regard it as irrelevant and do not care about it. But it will undoubtedly come to pass. Then Allah states that He is Able to do all things and He encompasses all things, and it is very easy for Him to bring about the Hour:
أَلاَ إِنَّهُ بِكُلِّ شَىْءٍ مُّحِيطُ
(Verily, He is surrounding all things!) means, all of creation is subject to His control, dominion and knowledge; He is controlling all things by His power, so whatever He wills happens, and whatever He does not will does not happen, and there is no god worthy of worship apart from Him. This is the end of the Tafsir of Surah Fussilat. To Allah be praise and blessings.