﴿ وَقَالُوا۟ قُلُوبُنَا غُلْفٌۢ ۚ بَل لَّعَنَهُمُ ٱللَّهُ بِكُفْرِهِمْ فَقَلِيلًۭا مَّا يُؤْمِنُونَ ﴾

“But they say, "Our hearts are already full of knowledge." Nay, but God has rejected them because of their refusal to acknowledge the truth: for, few are the things in which they believe.”

And they say to the Prophet mockingly ‘Our hearts are encased’ ghulf is the plural of aghlaf that is to say wrapped up in covers and cannot comprehend what you say; God exalted be He says Nay bal introduces the rebuttal but God has cursed them removed them far from His mercy and degraded them when they rejected the messengers for their unbelief which is not the result of anything defective in their hearts; and little will they believe fa-qalīlan mā yu’minūn the mā here is extra emphasising the ‘littleness’ involved that is their belief is minimal.