"O you who have believed, fear Allah and give up what remains of interest [riba]," meaning: what remains of what they practiced in terms of interest during the pre-Islamic period (Jahiliyyah), that they should not take it. As for what they took before their Islam, it is lawful for them. "If you should be believers," meaning: since you are believers.
﴿يَا أَيهَا الَّذِينَ آمَنُوا اتَّقُوا اللَّهَ وَذَرُوا مَا بَقِي من الرِّبَا﴾ يَعْنِي: مَا بَقِي مِمَّا أربوا فِيهِ فِي الْجَاهِلِيَّة أَلا يأخذوه، وَمَا أخذُوا قبل إسْلَامهمْ فَهُوَ حَلَال لَهُم ﴿إِنْ كُنْتُمْ مُؤْمِنِينَ﴾ يَعْنِي: إِذْ كُنْتُم مُؤمنين.