Turning this over in my mind, I've realised that the problem extends beyond politics and into every area of life. Why get involved in culture, if culture will always ultimately be corrupt and belong to the passing form of this world? Why be involved in poverty relief or development, given that we will always have the poor with us? Perhaps most fundamentally, why seek personal holiness, when I will always be fallen and never achieve perfection this side of my resurrection?
A few answers spring to mind. Perhaps we could say that there is value in the effort itself, regardless of the result. That may be, but I struggle to see wherein the value lies, and that in itself makes this no good as a motivating force. We could perhaps argue that small successes count for something, but I am not sure that this is obviously the case. It is certainly true to argue that culture and the like are carried, somehow, through the end and into the new creation, but the process is obscure in its details, and I don't feel all that motivated by it.
I've arrived at the conclusion that the motive is witness. I don't seek to change society because I think it can be ultimately fixed, but as a witness to the fact that in principle it has already been ultimately fixed in the Kingdom of Jesus Christ. I don't seek to exercise my creativity because I believe I can make a perfect culture, but as a bearing witness to the perfect culture that already exists in principle through the death and resurrection of Christ. And I don't seek personal holiness because I can create it through my own efforts, but as my witness to the fact that complete righteousness is already mine in the risen Lord Jesus.
All my life is just saying 'yes', in every sphere in which I have influence, to what He has done in every sphere of existence.