(… يَا أَيُّهَا الَّذِينَ آمَنُوا ) (O you who have believed …): The believers are addressed because there is a contract of faith between the believers and the Lord. Thus, the believers must obey the orders and fulfil the obligations by Allah (swt); and they are addressed in this case. Consequently, whenever the Almighty says: (O you who have believed) there follows a fiducial duty or an obligation:

{ يَا أَيُّهَا الَّذِينَ آمَنُوا كُتِبَ عَلَيْكُمُ الصِّيَامُ}

(O you who have believed, decreed upon you is fasting.) (Al-Baqara: 183)

{ يَا أَيُّهَا الَّذِينَ آمَنُوا أَنفِقُوا مِمَّا رَزَقْنَاكُم}

(O you who have believed, spend from that which We have provided for you.) (Al-Baqara: 254)

{ يَا أَيُّهَا الَّذِينَ آمَنُوا لَا يَحِلُّ لَكُمْ أَن تَرِثُوا النِّسَاءَ كَرْهًا}

(O you who have believed, it is not lawful for you to inherit women by compulsion.) (An-Nisaa: 19)

The believers were even addressed when it was about banning usury because usury was so common in the society prior to Islam.

(… اتَّقُوا اللَّهَ …) (…And fear Allah; indeed …): Fearing Allah, piety, is to fear the attributes of majesty of Allah (swt) such as the Majestic, the Overpowering, the Avenger, and so on. Fearing Allah (swt) means that you follow His orders and avoid what He has forbidden.

(… وَذَرُوا مَا بَقِيَ مِنَ الرِّبَا …) (…and give up what remains [due to you] of interest …): ‘and’ is a conjunction, and ‘give up’ here means that you should leave what remains of usury because some people were still having something left therefrom.

( إِن كُنتُم مُّؤْمِنِينَ … ) (…if you should be believers): ‘If’ is conditional. If you are not believers, then it’s your own business and your own account will be in the afterlife. As for the believers, they must fulfil duties whether they know the reason behind then or not. Believers follow the orders of the Almighty, so it’s good if they know the wisdom therein. If no, then they worship by following the orders of the Almighty.