What is religion (dīn) according to the Qur'an?

لَآ إِكْرَاهَ فِى ٱلدِّينِ ۖ قَد تَّبَيَّنَ ٱلرُّشْدُ مِنَ ٱلْغَىِّ ۚ فَمَن يَكْفُرْ بِٱلطَّـٰغُوتِ وَيُؤْمِنۢ بِٱللَّهِ فَقَدِ ٱسْتَمْسَكَ بِٱلْعُرْوَةِ ٱلْوُثْقَىٰ لَا ٱنفِصَامَ لَهَا ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ

There is no compulsion in religion. The right direction is henceforth distinct from error. And he who rejecteth false deities and believeth in Allah hath grasped a firm handhold which will never break. Allah is Hearer, Knower.

إِنَّ ٱلدِّينَ عِندَ ٱللَّهِ ٱلْإِسْلَـٰمُ ۗ وَمَا ٱخْتَلَفَ ٱلَّذِينَ أُوتُوا۟ ٱلْكِتَـٰبَ إِلَّا مِنۢ بَعْدِ مَا جَآءَهُمُ ٱلْعِلْمُ بَغْيًۢا بَيْنَهُمْ ۗ وَمَن يَكْفُرْ بِـَٔايَـٰتِ ٱللَّهِ فَإِنَّ ٱللَّهَ سَرِيعُ ٱلْحِسَابِ

Lo! religion with Allah (is) the Surrender (to His Will and Guidance). Those who (formerly) received the Scripture differed only after knowledge came unto them, through transgression among themselves. Whoso disbelieveth the revelations of Allah (will find that) lo! Allah is swift at reckoning.

وَمَن يَبْتَغِ غَيْرَ ٱلْإِسْلَـٰمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِى ٱلْـَٔاخِرَةِ مِنَ ٱلْخَـٰسِرِينَ

And whoso seeketh as religion other than the Surrender (to Allah) it will not be accepted from him, and he will be a loser in the Hereafter.

فَأَقِمْ وَجْهَكَ لِلدِّينِ حَنِيفًا ۚ فِطْرَتَ ٱللَّهِ ٱلَّتِى فَطَرَ ٱلنَّاسَ عَلَيْهَا ۚ لَا تَبْدِيلَ لِخَلْقِ ٱللَّهِ ۚ ذَٰلِكَ ٱلدِّينُ ٱلْقَيِّمُ وَلَـٰكِنَّ أَكْثَرَ ٱلنَّاسِ لَا يَعْلَمُونَ

So set thy purpose (O Muhammad) for religion as a man by nature upright - the nature (framed) of Allah, in which He hath created man. There is no altering (the laws of) Allah's creation. That is the right religion, but most men know not -

۞ قَالَتِ ٱلْأَعْرَابُ ءَامَنَّا ۖ قُل لَّمْ تُؤْمِنُوا۟ وَلَـٰكِن قُولُوٓا۟ أَسْلَمْنَا وَلَمَّا يَدْخُلِ ٱلْإِيمَـٰنُ فِى قُلُوبِكُمْ ۖ وَإِن تُطِيعُوا۟ ٱللَّهَ وَرَسُولَهُۥ لَا يَلِتْكُم مِّنْ أَعْمَـٰلِكُمْ شَيْـًٔا ۚ إِنَّ ٱللَّهَ غَفُورٌ رَّحِيمٌ

The wandering Arabs say: We believe. Say (unto them, O Muhammad): Ye believe not, but rather say "We submit," for the faith hath not yet entered into your hearts. Yet, if ye obey Allah and His messenger, He will not withhold from you aught of (the reward of) your deeds. Lo! Allah is Forgiving, Merciful.

لَكُمْ دِينُكُمْ وَلِىَ دِينِ

Unto you your religion, and unto me my religion.