Al-Muminoon 23:118

Shirk is the Worst form of Wrong, its Practitioner shall never succeed. Allah threatens those who associate anything else withHim and worship anything with Him. He informs that those whoassociate others with Allah:

(of whom he has no proof), meaning no evidence for what he says. Then Allah says:

(And whoever invokes, besides Allah, any other god, of whom he has no proof;) this is a conditional sentence, whose fulfilling clause is:

(then his reckoning is only with his Lord.) meaning, Allah will call him to account for that. Then Allah tells us:

(Surely, disbelievers will not be successful.) meaning, they will not be successful with Him on the Day of Resurrection; they will not prosper or be saved.

(And say: "My Lord! Forgive and have mercy, for You are the best of those who show mercy!") Here Allah is teaching us to recite this supplication, for forgiveness, in a general sense, means wiping away sins and concealing them from people, and mercy means guiding a person and helping him to say and do good things.

رَّ‌بِّ اغْفِرْ‌ وَارْ‌حَمْ (My Lord, forgive and have mercy - 23:118). Here the object of the forgiveness and having mercy has not been mentioned. What is there to be forgiven and to whom is mercy to be shown? And this seems to suggest a general prayer for forgiveness which would include deliverance from harm and suffering, and a general prayer for mercy which would include the grant of every wish. Since protection from bodily harm and acquisition of legitimate gains is the object of all human activity, this prayer covers both these aspects. (Mazhari) And the reason why the Holy Prophet ﷺ was commanded to recite this prayer of forgiveness and mercy, even tough he is impeccable, is to impress upon his followers the special merit of this prayer. (Qurtubi)

Surah Al-Mu'minun began with the verse قَدْ أَفْلَحَ الْمُؤْمِنُونَ (Success is really attained by the believers - 23:1) and ends withإِنَّهُ لَا يُفْلِحُ الْكَافِرُ‌ونَ (Surely, the infidels will not achieve success - 23:117) which shows that success will come to believers only and that the unbelievers will have no share of it.