(… فَإِن لَّمْ تَفْعَلُوا ) (And if you do not …): Some doesn’t want to do that (give up on usury). The proof is the capitalist setup prevailing all over the world nowadays.

(… فَأْذَنُوا بِحَرْبٍ مِّنَ اللَّهِ وَرَسُولِهِ ) (…Then be informed of a war [against you] from Allah and His Messenger …): This is very dreadful because it is the only time that Allah (swt) has such phrase.

{ وَمَا يَعْلَمُ جُنُودَ رَبِّكَ إِلَّا هُوَ}

(And none knows the soldiers of your Lord except Him.) (Al Muddaththir: 31)

You do not know what He does, He destroys money, takes health … etc. Moreover, you may suffer from sins, adversity, calamities and burdens and will be held accountable on the Day of Resurrection:

{ وَفِي الْآخِرَةِ عَذَابٌ شَدِيدٌ}

(And in the Hereafter is severe punishment.) (Al-Hadid: 20)

(… فَأْذَنُوا …) (…then be informed …): Get informed through your sense of hearing:

{ إِنَّ السَّمْعَ وَالْبَصَرَ وَالْفُؤَادَ كُلُّ أُولَٰئِكَ كَانَ عَنْهُ مَسْئُولًا}

(Indeed, the hearing, the sight and the heart – about all those [one] will be questioned.) (Al-Israa: 36)

So, be informed of a war where you too powerless to confront your opponents, Allah (swt) and His Messenger.

( وَإِن تُبْتُمْ فَلَكُمْ رُءُوسُ أَمْوَالِكُمْ لَا تَظْلِمُونَ وَلَا تُظْلَمُونَ … ) (…But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged): Why? Because He has guaranteed your capital and prevented you from increasing it through usury or interest that are forbidden by Allah (swt). Then, repentance is required. In addition, he whoever deals in usury has committed a deadly sin. Prophet Muhammad (saws) says:

“‘avoid the seven destructive things;’ it was asked: (by those present) “What are they, O messenger of Allah ?”He replied, “Associating anyone or anything with Allah in worship; practising sorcery, killing of someone without a just cause whom Allah has forbidden, devouring the property if an orphan, eating of usury, fleeing from the battlefield and slandering chaste women who never even think of anything touching chastity and are good believers[1].”

[1] Hadith no. 2615, Chapter: His Almighty said {Indeed, those who devour the property of orphans unjustly are only consuming into their bellies fire. And they will be burned in a Blaze(10)} (An-Nisaa), Wills and Testaments (Wasaayaa), Sahih Al-Bukhari