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:

لاَ بُرْهَانَ لَهُ

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

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.