why we can and should try to change the world

I believe we live in a world that is presently at once already redeemed and in the process of being redeemed - that we are at once beloved of God and in the process of becoming 'beloved of God'. Maybe like a marriage (although I have no experience of such): where you love the person that you marry, but neither of you stay at that point if your marriage is going to survive. You continue to love each other, every day, every hour, every action-word-thought.

As such, I think that while Adam tilled the ground and it produced thistles and thorns and rocks, in this time of redemption, we have the tools and the knowledge to make that work easier, simpler, less consuming. It still needs to be done, it still takes effort, and there's plenty of failure along the way. But in the same way that freedom for slaves and voices for women and the idea that all people are equal only started being a thing after Christians rolled up their sleeves and started working for change at the ground level, learning easier ways to do things, tools that could be used for the benefit of many, even things like democracy and so forth - all that had to come once God had set the world to rights. Once God made it possible for us to help Him set the world to rights.

I think that we had to know redemption from Christ before we could start redeeming the world.

And no, it won't be done before Christ comes again; but that's no reason to sit back and say the world is doomed.

I guess it's a variant of social justice Christianity (and man, the number of times I've heard Christians talk about social justice like it's a dirty thing to want a better, fairer society in this world) - the idea that we can make things better for the vulnerable and fragile, and that we also should.