Answer: Faith in humanity is used to express hope (or lack thereof) in our ability as a species to be good or thrive. If you see something that makes you "lose hope for humanity," it is usually a cruel or malicious act that makes one question whether we as a species will survive, or at least have a good impact while we're here. Things that give hope for humanity are usually altruistic acts that make people think "we just might be alright." If you think humanity is a dumpster fire of shitty people and we are inevitably going to bomb ourselves to extinction, you've lost faith in humanity. If you think people are good in their core and, as bad as the bad apples are, we are generally good to each other, if you think that we can build something greater than we are now and are going to be alright, you have faith in humanity. These statements are generally made in hyperbole to express how one is feeling in the moment, and usually isn't a description of a more permanent state of thinking. At least not colloquially.

As far as origin, no clue.