One answer to that is from CS Lewis book the Silver Chair. You might not know the story but at this point two children and a Marshwiggle have just rescued the Prince from the Witches underground castle, and then she turns up and tried to bewitch them into believing there is no other world to go back to. This is Puddleglum's response:
.My answer to the question is that I would live this life even if there is no eternal life to look forward to. Looking at it from a totally non-spiritual basis, if the only thing that raises mankind above animals is morality, and the only moral purpose for life is to do good to others, then I still do not know a better way to "do good" than the way I am living...Suppose we have only dreamed, or made up, all those things- trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things things seem a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that's a funny thing, when you come to think of it,. We're just babies making up a game, is you're right. But four babies playing a game can make a play world that licks your real world hollow. That's why I'm going to stand by the play-world. I'm on Aslan's side even if there isn't any Aslan to lead it. I'm going to live as like a Narnian as I can even if there isn't any Narnia. So, thanking you kindly for our supper, if the two gentlemen and the young lady are ready, we're leaving your court at once and setting out in the dark to spend our lives looking for Overland. Not that our lives will be very long I should think, but that's small loss if the world's as a dull a place as you say.”
As an individual I might not make any great contrubutions, but as a group of people working together as the church I see the "lost ones" of our society restored and finding value as a person, worth in society and healing in their souls.
I could be an ecological martyr- recycle all my rubbish, eat only organic fair-traded food, refuse to shop in supermarkets etc but I think I would jsut get side-tracked in doing so. It is possible to champion such causes and still be essentially a selfish/hurt/unloving/unhealed person.
I totally believe that it is only in God, through Jesus Christ, that people are able to become who they are really meant to be. When God sent Jesus to die on the cross he was not just on a rescue mission- sending out a lifeboat to populate heaven. God intends us to start our relationship with Him now, here on earth, to give us what He always intended us to have in Eden, to make us fully who He wanted us to be.