It is solved by a democratic system that defines truth as "mutually observable phenomena", defines good as "the wishes of the people", and allows individuals to engage in free dialogue as a replacement for violence.
Good outnumbers bad, so the good will win, unless both sides think they're good in a 50/50 split.
This can happen even in that ideal society, because 50% of the individuals will eventually decide to have fundamentally different goals as the other 50%. In which case, I don't think we should hold that society together by force, but rather provide a mechanism for it to peacefully split into two, precisely to uphold the democratic principle of respecting the wishes of every individual.
Suppose half of those people are mistaken in a collective delusion, and their goals are in actuality aligned with the other half, but the other half have just failed so spectacularly at enlightening them (or perhaps the delusional half are so spectacularly delusional that they're impossible to enlighten). In this rare case of a perfect failure, they will quickly realize after the split and want to get back together, because reality is a harsh judge, and its judgements are ultimate.