Surah Al-An'am, Ayah 147

If they deny you, O Messenger, and do not accept what you brought from your Lord, then say to them in encouragement: Your Lord is One, possessor of vast mercy. It is because of His mercy that He has given you relief and has not yet taken you to task by punishing you. Also say to them as a warning: His punishment is not averted from those who commit sins.

If they deny you say: "Your Lord is the Owner of vast mercy, and never will His wrath be turned back from the people who are criminals. (147)

Allah says, if your opponents among the idolators, Jews and their likes reject you, O Muhammad,

فَقُل رَّبُّكُمْ ذُو رَحْمَةٍ وَاسِعَةٍ

(Say: "Your Lord is the Owner of vast mercy...") encouraging them to seek Allah's vast mercy and follow His Messenger ﷺ,

وَلَا يُرَدُّ بَأْسُهُ عَنِ الْقَوْمِ الْمُجْرِمِينَ

(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:

إِنَّ رَبَّكَ سَرِيعُ الْعِقَابِ وَإِنَّهُ لَغَفُورٌ رَّحِيمٌ

(Surely, your Lord is swift in retribution, and certainly He is Oft-Forgiving, Most Merciful.)[6:165] Allah also said,

وَإِنَّ رَبَّكَ لَذُو مَغْفِرَةٍ لِّلنَّاسِ عَلَىٰ ظُلْمِهِمْ وَإِنَّ رَبَّكَ لَشَدِيدُ الْعِقَابِ

(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

نَبِّئْ عِبَادِي أَنِّي أَنَا الْغَفُورُ الرَّحِيمُ - وَأَنَّ عَذَابِي هُوَ الْعَذَابُ الْأَلِيمُ

(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

غَافِرِ الذَّنبِ وَقَابِلِ التَّوْبِ شَدِيدِ الْعِقَابِ

(The Forgiver of sin, the Acceptor of repentance, the Severe in punishment.)[40:3] and,

إِنَّ بَطْشَ رَبِّكَ لَشَدِيدٌ - إِنَّهُ هُوَ يُبْدِئُ وَيُعِيدُ - وَهُوَ الْغَفُورُ الْوَدُودُ

(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.