Allah says: leave Me with the one I created alone, and gave him abundant wealth, and sons present with him, and made life smooth for him; then he wants Me to give him more. No! He has been stubborn toward Our signs (74:11-16). Ibn Kathir explains: he came out of his mother’s womb alone, with no wealth and no child, then Allah provided for him; and he met these blessings with ingratitude, denying Allah’s signs and calling them the words of a human being.
ذَرْنِي وَمَنْ خَلَقْتُ وَحِيدًا (سورة المدثر: 11)

قال تعالى: (ذرني ومن خلقت وحيدا وجعلت له مالا ممدودا وبنين شهودا ومهدت له تمهيدا ثم يطمع أن أزيد كلا إنه كان لآياتنا عنيدا) (المدثر 74: 11-16)، قال ابن كثير: أي خرج من بطن أمه وحده لا مال له ولا ولد، ثم رزقه الله، فكفر بأنعم الله وبدلها كفرا، وقابلها بالجحود بآيات الله والافتراء عليها، وجعلها من قول البشر.

Allah says: he thought and deliberated; may he be destroyed, how he deliberated (74:18-19). Ibn Kathir explains: he pondered what to say about the Quran when he was asked about it, thinking what words he could invent. Then he said: this is nothing but magic handed down; this is nothing but the words of a human being (74:24-25), which Ibn Kathir explains as his claim that it was magic Muhammad passed on from others before him.
إِنَّهُ فَكَّرَ وَقَدَّرَ (سورة المدثر: 18)

قال تعالى: (إنه فكر وقدر فقتل كيف قدر) (المدثر 74: 18-19)، قال ابن كثير: أي تروى ماذا يقول في القرآن حين سئل عن القرآن، ففكر ماذا يختلق من المقال. ثم قال: (إن هذا إلا سحر يؤثر إن هذا إلا قول البشر) (المدثر 74: 24-25)، قال ابن كثير: أي هذا سحر ينقله محمد عن غيره عمن قبله ويحكيه عنهم.

The threat follows: I will drive him into Saqar (74:26). Ibn Kathir explains: I will cover him in it from every side. How often Saqar is named in the Quran is explained in how many times Saqar is mentioned in the Quran; an opponent of the Prophet whom the Quran names outright is covered in which surah names an opponent of the Prophet.
سَأُصْلِيهِ سَقَرَ (سورة المدثر: 26)

قال تعالى: (سأصليه سقر) (المدثر 74: 26)، قال ابن كثير: أي سأغمره فيها من جميع جهاته.