The Idolators of Every Nation made a Mockery of their Messengers
Consoling His Messenger for the rejection of the disbelieving Quraysh, Allah says that He has sent Messengers before him to the nations of the past, and no Messenger came to a nation but they rejected him and mocked him. Then He tells him that He lets disbelief enter the hearts of those sinners who are too stubborn and too arrogant to follow His guidance.
كَذَلِكَ نَسْلُكُهُ فِى قُلُوبِ الْمُجْرِمِينَ
(Thus We allow it to enter the hearts of the guilty.) Anas and Al-Hasan Al-Basri said that this referred to Shirk.
وَقَدْ خَلَتْ سُنَّةُ الاٌّوَّلِينَ
(and already the example of the ancients has gone forth.) meaning the destruction wrought by Allah on those who rejected His Messengers, and how He saved His Prophets and their followers in this world and in the Hereafter, is well known.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
The majority of the translators and commentators are of the opinion that the pronoun `it' in ".... cause it" in v. 12, refers to their "mockery", occurring in v. 11, and in “would not believe in it” in v. 13, refers to the "Admonition" occurring in v. 9. Then vv. 12-13 will be rendered like this: "Thus we cause mockery enter into the hearts of the criminals and they do not believe in the Admonition.... " Though grammatically there is nothing wrong with this version, yet our version of "it" will be better even grammatically. According to this v. 12 will mean, "When the Admonition enters into the hearts of the believers it gives them peace of mind and comfort of heart. But when the same enters into the hearts of the criminals it becomes a hot rod, and burns their minds and hearts.”