Imaam Ibn Seereen (rahimahullaah) said:

There is nothing more frightening than this Ayah:

[وَمِنَ النَّاسِ مَنْ يَقُولُ آمَنَّا بِاللَّهِ وَبِالْيَوْمِ الْآخِرِ وَمَا هُمْ بِمُؤْمِنِينَ]

”And of mankind, there are some who say: “We believe in Allah and the Last Day” while in fact they believe not.” [Q 2:8]

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Source: [Fat-hul Baari of Ibn Rajab 1/197]