So if they deny you, [O Muḥammad], say, "Your Lord is the possessor of vast mercy; but His punishment cannot be repelled from the people who are criminals."
(and never will His wrath be turned back from the people who are criminals.) discouraging them from defying the Messenger, the Final Prophet, Muhammad ﷺ. Allah often joins encouragement with threats in the Qur'an. Allah said at the end of this Surah:
(But verily, your Lord is full of forgiveness for mankind in spite of their wrongdoing. And verily, your Lord is (also) severe in punishment.) 13:6, and
(Declare unto My servants, that truly, I am the Oft-Forgiving, the Most Merciful. And that My torment is indeed the most painful torment.) 15:49-50, and
(Verily, the punishment of your Lord is severe and painful. Verily, He it is Who begins and repeats. And He is Oft-Forgiving, full of love.)85:12-14. There are many other Ayat on this subject.
Ma'arif al-Qur'an
Mufti Muhammad ShafiOn ayahs 142–150
Commentary
Mentioned in the previous verses was how astray the disbelievers of Makkah had gone when they had taken their self-carved, lifeless, insensate idols as partners in the Divinity of Allah as they made them share in animals created by Him and in many other blessings which were bestowed on them. Their transgression went to the limits when they would take out charities from these blessings as an act of worship, setting aside a portion for Allah and another for their idols. Then, they would employ different excuses to take out what belonged to the portion set aside for Allah and put it in the portion reserved for the idols. Similarly, there were many other ignorance-based arbitrary customs which they had given legal status.
According to the divine Shariah (Islamic regulations), the truly forbidden things have always been the same as mentioned in the foregoing verse, viz. carrion, out-pouring blood, the meat of pigs and animals slaughtered in the name of any thing or being other than God. Anything else which is forbidden falls into a sub-group of the above-mentioned as a matter of detail. But along with this, among God’s prohibitory laws, another principle is at work, namely, that when a community blessed with His Book adopts the way of arrogance instead of obedience, it is subjected to new difficulties by way of punishment, i.e. such things as were not originally forbidden them in the Divine Shariah are now banned. One way in which this change comes about is that in such a community, leaders arise who are completely ignorant of the reality of the religion. They are conversant only with its outward formalities. The meticulousness which they should apply to the meaningful realities of religion is reserved by them for outward formalities and rites. The result is that unnecessary hair-splitting over externals becomes a set pattern in religion. Such leaders devise self-made outward standards of religion. By a process of extreme exaggeration, they make a simple commandment complicated and a legitimate thing unlawful. For instance, when arrogance set in among the Jews, certain scholars arose among them who, by their hair-splitting arguments, made the rule that for an animal to be deemed fit to eat, it should possess two qualities—one, that its hooves should be cloven or divided, and second, that it should be a cud-chewing animal. Even if one of these conditions was not fulfilled, that animal, according to them, was forbidden. Due to this self-imposed condition, even animals like the camel and the rabbit, etc., were unnecessarily banned. Similarly, by taking up an extreme position about the importance attached to claws, they unnecessarily placed ostriches, swans and ducks in the forbidden category. These unnatural restrictions narrowed their lives, whereas God had intended to give them wider latitude. After rejecting the Truth, man is not immediately seized upon by God. He finds himself free and his life as full as before. For this reason, he erroneously imagines that he is not going to lose anything by not accepting the Truth. He forgets that if he remains safe, it is simply because of the merciful forbearance of God. In spite of man’s arrogance, God gives him the opportunity to redeem himself till the very last moment. Finally, when he has not changed his behaviour, all of a sudden God swoops down on him—sometimes in this world and sometimes both in this world and the Hereafter.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.