Allah commands His Messenger to inform all of creation that no one among the dwellers of heaven and earth knows the Unseen, except Allah.
إِلاَّ اللَّهُ
(except Allah) This is an absolute exception, meaning that no one knows this besides Allah, He is alone in that regard, having no partner in that knowledge. This is like the Ayat:
وَعِندَهُ مَفَاتِحُ الْغَيْبِ لاَ يَعْلَمُهَآ إِلاَّ هُوَ
(And with Him are the keys of the Unseen, none knows them but He) (6:59).
إِنَّ اللَّهَ عِندَهُ عِلْمُ السَّاعَةِ وَيُنَزِّلُ الْغَيْثَ
(Verily, Allah, with Him is the knowledge of the Hour, He sends down the rain) (31:34). until the end of the Surah. And there are many Ayat which mention similar things.
وَمَا يَشْعُرُونَ أَيَّانَ يُبْعَثُونَ
(nor can they perceive when they shall be resurrected.) That is, the created beings who dwell in the heavens and on earth do not know when the Hour will occur, as Allah says:
ثَقُلَتْ فِى السَّمَـوَتِ وَالاٌّرْضِ لاَ تَأْتِيكُمْ إِلاَّ بَغْتَةً
(Heavy is its burden through the heavens and the earth. It shall not come upon you except all of a sudden) (7: 187). meaning, it is a grave matter for the dwellers of heaven and earth.
بَلِ ادَرَكَ عِلْمُهُمْ فِى الاٌّخِرَةِ بَلْ هُمْ فِى شَكٍّ مِّنْهَا
(Nay, their knowledge will perceive that in the Hereafter. Nay, they are in doubt about it.) means their knowledge and amazement stops short of knowing its time. Other scholars read this with the meaning "their knowledge is all the same with regard to that," which reflects the meaning of the Hadith in Sahih Muslim which states that the Messenger of Allah ﷺ said to Jibril, when the latter asked him when the Hour would come: s
«مَا الْمَسْؤُولُ عَنْهَا بِأَعْلَمَ مِنَ السَّائِل»
(The one who is being asked about it does not know any more than the one who is asking.) In other words, they were both equal in the fact that their knowledge did not extend that far.
بَلْ هُمْ فِى شَكٍّ مِّنْهَا
(Nay, they are in doubt about it.) This refers to the disbelievers in general as Allah says elsewhere:
وَعُرِضُواْ عَلَى رَبِّكَ صَفَا لَّقَدْ جِئْتُمُونَا كَمَا خَلَقْنَـكُمْ أَوَّلَ مَرَّةٍ بَلْ زَعَمْتُمْ أَلَّن نَّجْعَلَ لَكُمْ مَّوْعِدًا
(And they will be set before your Lord in rows, (and Allah will say:) "Now indeed, you have come to Us as We created you the first time. Nay, but you thought that We had appointed no meeting for you (with Us). ") (18:48) i.e., the disbelievers among you. By the same token, Allah says here:
بَلْ هُمْ فِى شَكٍّ مِّنْهَا
(Nay, they are in doubt about it.) meaning, they doubt that it will come to pass.
بَلْ هُم مِّنْهَا عَمُونَ
(Nay, they are in complete blindness about it.) They are blind and completely ignorant about it.
وَقَالَ الَّذِينَ كَفَرُواْ أَءِذَا كُنَّا تُرَاباً وَءَابَآؤُنَآ أَءِنَّا لَمُخْرَجُونَ - لَقَدْ وُعِدْنَا هَـذَا نَحْنُ وَءَابَآؤُنَا مِن قَبْلُ إِنْ هَـذَآ إِلاَّ أَسَـطِيرُ الاٌّوَّلِينَ