Only some???? ROTFL!
Belloc was an Englishman whose father was French. In fact, he stood for election to the British House of Commons at one point (not sure if he won).
I have no idea what a "Catholic supremacist" is. Is that a Catholic with the temerity and bad manners to actually believe that his faith is true? I'll proudly plead "guilty as charged" to that, then.
He contended that Protestantism and Muhammadanism are both just heretical deviations of the Truth.
Obviously, to a Catholic, Protestantism and Muhammadanism [sic] are both in error. Both also contain some elements of truth.