The Polytheism and Disbelief of the Christians
Allah states that the Christians are disbelievers because of their claim that `Isa, son of Maryam, one of Allah's servants and creatures, is Allah. Allah is holier than what they attribute to Him. Allah then reminds them of His perfect ability over everything and that everything is under His complete control and power,
قُلْ فَمَن يَمْلِكُ مِنَ اللَّهِ شَيْئاً إِنْ أَرَادَ أَن يُهْلِكَ الْمَسِيحَ ابْنَ مَرْيَمَ وَأُمَّهُ وَمَن فِى الاٌّرْضِ جَمِيعاً
(Say: "Who then has the least power against Allah, if He were to destroy the Messiah, son of Maryam, his mother, and all those who are on the earth together") Therefore, if Allah wills to do that, who would be able to stop Him or prevent Him from doing it Allah then said,
وَللَّهِ مُلْكُ السَّمَـوَتِ وَالاٌّرْضِ وَمَا بَيْنَهُمَا يَخْلُقُ مَا يَشَآءُ
(And to Allah belongs the dominion of the heavens and the earth, and all that is between them. He creates what He wills.) All things in existence are Allah's property and creation and He is able to do everything. He is never asked about what He does with His power, domain, justice and greatness so this refutes the Christian creed, may Allah's continued curses be upon them until the Day of Resurrection.
Refuting the People of the Book's Claim that they are Allah's Children
Allah then refutes the Christians' and Jews' false claims and lies,
وَقَالَتِ الْيَهُودُ وَالنَّصَـرَى نَحْنُ أَبْنَاءُ اللَّهِ وَأَحِبَّاؤُهُ
(And the Jews and the Christians say, "We are the children of Allah and His loved ones.") They claim: "We are the followers of Allah's Prophets, who are His children, whom He takes care of. He also loves us." The People of the Book claim in their Book that Allah said to His servant Isra'il, "You are my firstborn." But they explained this statement in an improper manner and altered its meaning. Some of the People of the Book who later became Muslims refuted this false statement saying, "This statement only indicates honor and respect, as is common in their speech at that time." The Christians claim that `Isa said to them, "I will go back to my father and your father," meaning, my Lord and your Lord. It is a fact that the Christians did not claim that they too are Allah's sons as they claimed about `Isa. Rather this statement by `Isa only meant to indicate a closeness with Allah. This is why when they said that they are Allah's children and loved ones, Allah refuted their claim,
قُلْ فَلِمَ يُعَذِّبُكُم بِذُنُوبِكُم
(Say, "Why then does He punish you for your sins") meaning, if you were truly as you claim, Allah's children and loved ones, then why did He prepare the Fire because of your disbelief, lies and false claims
بَلْ أَنتُمْ بَشَرٌ مِمَّنْ خَلَقَ
(Nay, you are but human beings, of those He has created,) Allah states: you are just like the rest of the children of Adam, and Allah is the Lord of all His creation,
يَغْفِرُ لِمَن يَشَآءُ وَيُعَذِّبُ مَن يَشَآءُ
(He forgives whom He wills and punishes whom He wills.) Allah does what He wills, there is none who can escape His judgement, and He is swift in reckoning.
وَللَّهِ مُلْكُ السَّمَـوَتِ وَالاٌّرْضِ وَمَا بَيْنَهُمَا
(And to Allah belongs the dominion of the heavens and the earth and all that is between them;) Therefore, everything is Allah's property and under His power and control,
وَإِلَيْهِ الْمَصِيرُ
(and to Him is the return.) In the end, the return will be to Allah and He will judge between His servants as He will, and He is the Most Just Who is never wrong in His judgment.
Tafsir Ibn Kathir (abridged, English).