The "doctrine" on MHFM website is delusionally dangerous armchair theology that condemns everyone besides them to Hell.
That's interesting, I've actually taken the time to read their material and listened to many of their debates and talks, and have never once read that or heard them say that.
Though much of what they say is accurate, I have found exactly what stevusmagnus says about them is true.
On the other hand, what they say about the the fewness of the number saved is oftentimes direct quotes of the saints. Just because it comes from the Dimond Brothers doesn't mean it is incorrect. That attitude is a real problem among Catholics today. Even if someone makes an absolutely orthodox Catholic declaration, if it comes from certain personalities, the very declaration itself is condemned.