I would say that Christianity does put humanity first.
Christianity is about Christ. You need to keep your eye on him not the churches or people claiming to be Christian. People aren’t perfect, but Christ was. He is the head of the church and I believe the bible comes right out and says he’s our example.( 1 Peter 2:21) I think he’s a pretty good example.
You said: There's one simple tenet that needs to be followed by all humans; treat every living thing with respect and dignity. You don't need anyone to tell you more. –I’ll agree with that.
Jesus left us with two commandments. The first is that we love God and the second, which the bible says is like the first, is that we love our neighbours as ourselves. The second sounds a lot like what you said. We may not know how to love our neighbor, but we probably know how we want to be treated- with respect and dignity. The first commandment says we are supposed love God above all else. Do you know who God is? The answer is found in 1John 4:8. God is love. That’s what the bible says. We are supposed to put love above all else- to temper the law and everything else we do with love.
We all die. The bible says that God so loved the world that he gave his only begotten son that all those who believe in him should not perish, but have everlasting life. Jesus came into this world to be more than an example for us to follow. He died so we could have everlasting life. I suppose if you don’t believe this then it might seem like jibberish. I believe it. We probably disagree. People disagree. I believe it because, speaking for myself, I have seen enough and experienced enough to believe there is more to this world and our lives than what we can just see and hear and I find that the wisdom in the bible continues to astonish me the more I study it.
“We are how we treat each other.”