All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
This includes the Basmalah [i.e. saying: Bismillaah Ar-Rahmaan Ar-Raheem (in the Name of Allaah, The Most Gracious, The Most Merciful)] which is part of a verse in Soorah An-Naml [Quran 27], as there is a consensus on this matter as well.
As regards something where there is difference of opinion, then one does not become a Kaafir if he denies that it is from the Quran, like denying that it (the Basmalah) is part of the verse in Soorah Al-Faatihah, or that it is a verse at the beginning of every Soorah, as there is a difference of opinion on it. Ibn Al-Arabi said: “If someone says it (the Basmalah) is not a verse at the beginning of every Soorah, he does not become a Kaafir as this is a matter of difference of opinion, but if someone says that it is not from the Quran, then he becomes a Kaafir as it is found in the verse of Soorah An-Naml [Quran 27]”
For more benefit, please refer to Fatwa 129445.
As regards the statement of the scholars that if someone doubts the Kufr (disbelief) of a Kaafir (disbeliever), he becomes a Kaafir himself, then this is in regard to those who are definitely disbelievers like the Jews and the Christians or others. Kashshaaf Al-Qinaa', authored by Al-Buhooti from the Hanbali School of jurisprudence, reads: “Whoever does not declare as a Kaafir someone who takes a religion other than Islam, like the Christians and the Jews, or doubts about their disbelief, or considers their creed to be correct, then he is a Kaafir...”
Allaah Knows best.