So, I've upvoted. I like the questions...
You shouldn't try to understand God, that is impossible.
Understanding what God wills us to do that is possible, and your questions hit on that point quite well.

~ "Before Christianity was globally adopted?"
History is a topic of importance. I have found that what one believes to be "Historical Fact" defines the religion they have.

~ "so they are condemned?"
God is more important than a few humans. Regardless, we can assume God has offered them the opportunity to receive forgiveness.

~ "What happens to all the other people of the world who have other religions? Does God just banish them all to hell?"
In life, we make all the choices (or at least feel that we do). Just because we lose that ability in death does not mean God is the one who placed us in "hell". Nor does it mean everyone who has not heard of Jesus is going to "hell". I put "hell" in quotes because I have less of an understanding as to what this "hell" is, that people talk about, than an understanding of who God is.

~ "Why must I have blind faith?"
Faith is required if the Truth of all things is to be impacted or moved by us. For one who does have Faith in Truth this is like asking; why must I believe in gravity or light?... You don't have to... Life is just better, if you do.
Truth does not yield to the "needs" of a petty human, and if it does I hope you come to an awareness, of how low it stooped, of how far back it bent, just so you could get your "proof".

~ "Christianity causes more division amongst the world than anything els!"
I'm glad you are being honest about this opinion. But you should think to yourself, who taught such an opinion to you. Yes, Jesus admitted his teachings would cause division. And, the Jews foretold that Jesus' teachings would cause division. But the division is caused by people picking specific teachings from Jesus(or the bible) and making those teachings the most important in the given Christian sect.
What was the most important teaching? Maybe what he said as he died?
"Father(Truth), forgive them"
So, we should forgive each other.
Now think, why does that create division?