The Call to repent before the Punishment comes

This Ayah is a call to all sinners, be they disbelievers or others, to repent and turn to Allah. This Ayah tells us that Allah, may He be blessed and exalted, will forgive all the sins of those who repent to Him and turn back to Him, no matter what or how many his sins are, even if they are like the foam of the sea. This cannot be interpreted as meaning that sins will be forgiven without repentance, because Shirk can only be forgiven for the one who repents from it. Al-Bukhari recorded that Ibn `Abbas, may Allah be pleased with him, said that some of the people of Shirk killed many people and committed Zina (illegal sexual acts) to a great extent; they came to Muhammad ﷺ and said, "What you are saying and calling us to is good; if only you could tell us that there is an expiation for what we have done." Then the following Ayat were revealed:

وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَـهَا ءَاخَرَ وَلاَ يَقْتُلُونَ النَّفْسَ الَّتِى حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ وَلاَ يَزْنُونَ

In verse 55, it was said: وَاتَّبِعُوا أَحْسَنَ مَا أُنزِلَ إِلَيْكُم (and follow the best of what has been sent down to you). The second phrase أَحْسَنَ مَا أُنزِلَ (the best of what has been sent down) means the Qur'an, and the whole Qur'an is but ahsan, the best - and the Qur'an can be called: اَحسَںَ و اکمل (best) also in the sense that out of all scriptures - Torah, Injil, Zabur - revealed from Allah Ta’ ala, the Qur'an is the best and the most perfect. (Qurtubi)