The greatest punishment for us is when our heart hardened. When heard get hard, it will lead us to all sins. And that lead us to many punishments.
Allah ﷻ says:
أَفَمَن شَرَحَ اللَّهُ صَدْرَهُ لِلْإِسْلَامِ فَهُوَ عَلَىٰ نُورٍ مِّن رَّبِّهِ ۚ فَوَيْلٌ لِّلْقَاسِيَةِ قُلُوبُهُم مِّن ذِكْرِ اللَّهِ ۚ أُولَٰئِكَ فِي ضَلَالٍ مُّبِينٍ
❝So is one whose breast Allah has expanded to (accept) Islam and he is upon a light from his Lord (like one whose heart rejects it)? Then woe to those whose hearts are hardened against the remembrance of Allah. Those are in manifest error.❞
al-Qurtubi reported: Malik ibn Dinar, may Allah have mercy on him, said, “A servant is not struck by a greater punishment than having a hard heart. Allah is not angry with people but that He removes mercy from their hearts.” [Tafsir al-Qurtubi]