In a nutshell, the theology is that this current physical reality is corrupt, sinful. The future state (a time TBD), is where all things will be made new. That is the hope that Christianity offers. The Christians of the first few centuries went though many horrific "tribulations". Life wasn't perfect for them, they knew and understood that. Yet, they persevered. They are the cloud of witnesses who have gone before us.
At the end of the day, either Jesus was a liar, lunatic, or he "Lord" to the Jews. You either believe what he says or you don't. It's really pretty simple. Christ never said that earth would be perfected for our earthly reality. So to answer the why evil exists question, technically can't, because humans can't possibly understand, fully, the entirety the mind of a god. All we know is that there is a hope of perfected eternity, where a just god, brings justice to all.