The foolish pagans were amazed by an affair that should not cause amazement!
Allaah (The Most High) said:
الٓرۚ تِلۡكَ ءَايَـٰتُ ٱلۡكِتَـٰبِ ٱلۡحَكِيمِ
أَكَانَ لِلنَّاسِ عَجَبًا أَنۡ أَوۡحَيۡنَآ إِلَىٰ رَجُلٍ۬ مِّنۡہُمۡ أَنۡ أَنذِرِ ٱلنَّاسَ وَبَشِّرِ ٱلَّذِينَ ءَامَنُوٓاْ أَنَّ لَهُمۡ قَدَمَ صِدۡقٍ عِندَ رَبِّہِمۡۗ
قَالَ ٱلۡڪَـٰفِرُونَ إِنَّ هَـٰذَا لَسَـٰحِرٌ۬ مُّبِينٌ
[الٓرۚ تِلۡكَ ءَايَـٰتُ ٱلۡكِتَـٰبِ ٱلۡحَكِيمِ] ”Alif-Lam-Ra. These are the Verses of the Book (the Qur’an) Al-Hakim.”
Imaam Sadi (rahimahullaah) said: It is this Qur’aan, which includes (all affairs) of wisdom, and rulings whose verses guides to the realities of Imaan, the divine legislated commandments and prohibitions, which all the Ummah (i.e. the Muslim nation) has received with pleasure, acceptance and submission. However, despite this many of them (i.e. the disbelievers among mankind) turn away and they do not know; so they wonder [أَنۡ أَوۡحَيۡنَآ إِلَىٰ رَجُلٍ۬ مِّنۡہُمۡ أَنۡ أَنذِرِ ٱلنَّاسَ ] ”That We have sent Our Inspiration to a man from among themselves (i.e. Prophet Muhammad) (saying): “Warn mankind” of Allaah’s punishment, and make them fear Allaah’s Anger, and remind them of Allaah’s Aayaat (i.e. his signs, revelations etc).
[وَبَشِّرِ ٱلَّذِينَ ءَامَنُوٓاْ ] ”And give good news to those who believe” with a truthful belief [أَنَّ لَهُمۡ قَدَمَ صِدۡقٍ عِندَ رَبِّہِم] ”That they shall have with their Lord the rewards of their good deeds”; That is: They will have abundant good recompense and accumulated rewards from their Lord due to the (good deeds) they have sent forth (for the hereafter) and the righteous truthful actions they have done. However, the disbelievers are amazed about this great man–a type amazement that led them to disbelieve in him; so [قَالَ ٱلۡڪَـٰفِرُونَ إِنَّ هَـٰذَا لَسَـٰحِرٌ۬ مُّبِين ] “The disbelievers say (about him): This is indeed an evident sorcerer”; That is: As they claimed, he has manifested clear sorcerer and it is not something hidden from anyone. This (statement of theirs) is due to their foolishness and obstinacy because they were amazed by an affair that should neither amaze them nor was it something strange to them; rather their ignorance and lack of awareness of what brings about well-being for them is what amazes one.
How is it that they did not believe in this noble Messenger who was sent among them and they knew him well, then they rejected his call and were eager to nullify his religion! But Allah will not allow except that His Light (i.e. the religion of Islamic monotheism which he gave all the Messengers) should be perfected even though the Kafirun (disbelievers) hate (it). 
 Slightly paraphrased; for further details see Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan