Shāykh Abdullāh b. Abdur-Rahmān Abā Butāyn رحمه الله said;
“And surely the Qur’ān gives evidence that Doubt in Usūl Al-Dīn is Kufr.
And doubt (الشك) is swaying between two things, like the one who does not approve the Messenger ﷺ with certainty nor reject him, And (the one who) does not approve the resurrection with certainty nor reject it and the likes of that.
Like the one who does not believe the obligation of the Salāh nor absence of it’s obligation, or does not believe in the prohibition of Zina nor believes the absence of it’s prohibition
– And this is Kufr by the Ijma of the ‘Ulamā, and there is no excuse for anyone who’s condition is like this – just because he does not understand the Hujjah of Allāh & his Bayan, because there is no excuse after reaching it (the Hujjah) even if he does not understand the Hujjah.”
- [Al-Durar Al-Saniyyah 10/366]