﴿ فَإِن كَذَّبُوكَ فَقُل رَّبُّكُمْ ذُو رَحْمَةٍۢ وَٰسِعَةٍۢ وَلَا يُرَدُّ بَأْسُهُۥ عَنِ ٱلْقَوْمِ ٱلْمُجْرِمِينَ ﴾

“And if they give thee the lie, say: "Limitless is your Sustainer in His grace; but His punishment shall not be averted from people who are lost in sin."”

So, if they deny you, [by saying] that God is the Lord of all-embracing mercy and will not chastise us for our insolence, then say: 'Yea! Your Lord is the Lord of all-embracing mercy, but He is also the Lord of severe vanquishing whose mercy will not repel His might, from the sinning folk'. Rather, it may be that He has deposited His vanquishing in the form of His graciousness and His graciousness in the form of His vanquishing.