And follow the best of what was revealed to you from your Lord [i.e., the Qur’ān] before the punishment comes upon you suddenly while you do not perceive,
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:
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)
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 54–60
When those who possess sensitive hearts are blessed with a deep realisation of God (ma‘rifah), they start becoming worried as to what would happen about their past sins. Similarly, even after adopting a God-fearing way of life, a man is likely to display shortcomings from time to time, and his sensitive nature makes him worry again on this account. These feelings in some people can become so intense as to cause a deep sense of frustration. For such people, God has declared in His Book that they should firmly believe that their God is Forgiving and Merciful. He does not look at a man’s past but at his present. He does not look to his outward form but at his inner soul. He treats a man with broad-mindedness and not with petty-mindedness. That is why, when a man turns towards Him, He once again takes him into the fold of His Mercy, howsoever big a blunder the man might have committed.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.