Why is the phrase as popular as it is?
On another note, I also don’t quite understand what it means or implies. When someone doesn’t have faith in humanity, what does that mean?
Also, what is the humanity in the phrase? Do they mean all 8 billion people? Or is humanity understood here as something metaphysical?
Or you could have the realist approach that while people are capable of altruism and selfless acts, capable of thinking of others and placing them before themselves…
…they are nearly always still tethered to real needs for their own continued survival, the method of their attempts at altruism at often misguided due to ignorance of the total situation, most people are bound by social norms and customs, or ideologies of some kind that often justify overlooking some tragedies and actually perpetuating others, people also generally despise serious sacrifice toward a more indirect and necessary good thing, or a direct and obvious good thing that requires immense sacrifice…
…Etc.
People are complex, you can verify that with psychology, sociology, political science, etc.
Humans, individually and as assembled in various groups, have often stood by and done nothing as crimes and atrocities have occured, and of course those committing those crimes and atrocities are other humana.
A realist has no faith in humanity to do the right thing.
A realist has faith that humans will continue to be human.
Has nothing to do with OPs question or my answer, but to address a single tangent you’ve offered, yes, people are complex.
OP asked what it means to not have faith in humanity, and the person who responded to your comment had a nuanced take on the answer. Is that really a tangent?
(btw your top comment is a very good answer)