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?
According to Google book search, the phrase goes at least back to the 1800s. It’s interesting to see that spike between 1900 and 1950. I’d bet it’s related to the horrors of WW1.
As for what it means, other posters have answered: To have faith in humanity is to believe that humans on average have an inherent desire to do the right thing.