There are actually three classes in communism. The political elites, the slaves and the subhumans. The subhumans typically consist of Christians and capitalists.
Good point, you are absolutely right. The “forbidden” class are those who don’t believe in and refuse to participate in the Communist structure.
In a free, Capitalistic society, if one doesn’t believe in it, they are free to experiment with what works best in their personal lives such as creating or joining a commune*, kibbutz, or similar community. In Communist controlled society, if you don’t believe in the Communist system, you can’t freely choose another path for yourself, you are “forbidden”.
*note: a Commune isn’t truly Communist because it is a community of those freely choosing to take part. Not a dictated structure.