Of course it's not everything, but I don't think humanism on its own would work without an older religion as its "bootstrap".
I feel like if you squint a little you can see all the positive-memetic adaptations that get bolted on to organised religion.
A commandment to “worship no other god but me” is a classic example of something that seems less like profound revelation and more like an effective memetic adaptation.
UsefulChart's video on humanism may help. https://www.youtube.com/watch?v=UWhz3SXPWkg
Absolutely not, because the fitness, in an evolutionary sense, of those memes does not correlate with their truthfulness but with how easy they are to believe because they appeal to our fears, hopes and dreams.
Take that "life after death" trope. The hard truth is that there isn't any, when you die, everything that made up "you" will be gone for good. But no religion based on that truth would ever thrive because the thought is just too scary and bleak.
If he isn't, then he's a powerful (yet imaginary) social construct that has evolved over the past 4,000 years to encode the wisdom of many past generations through a set of rules and stories that happen to match up with the stories and rules of many other cultures. It's also an incredibly fast and low-cost way to reach a solution.
Either way, there's definitely some validity to the claims of any major religion.
The Atlantic slave trade started in the 1500s and ended in the 1800s. Even then, it was controversial and faces opposition from certain Church groups (while getting support from others).
Vilification of gays based on Christian religious grounds apparently goes back to the 11th century (based on a quick read of Wikipedia, TBF), and is only starting to come undone now. I don't really have an answer for this, it was unfortunate.