"Hope is one of the Theological virtues. This means that a continual looking forward to the eternal world is not (as some modern people think) a form of escapism or wishful thinking, but one of the things a Christian is meant to do."
Hope, in the Christian sense of the term according to Lewis, does not mean that we leave the world as it is because we're busy thinking and hoping for the next world and Something Better. The people who do the most good in this world are also, he says, those who think the most of the next. They are looking forward to the next world, to meeting their creator and having Him look at them and know everything that they've done in life and they're working to try and be 'worthy'.
Not that any of them think that they can earn their salvation, mind. Just that they don't want to be in front of God and have Him say, 'And what did you do with what I gave you?' and not have an answer except for, 'Wait to die and be with You.' The scenario brings to mind so many examples from the Bible, including the parable of the talents. God has given us things, including the entire world, and expects us to be good stewards of it. He expects us to improve things. He expects us to do something with what He's loaned us and that's why those who think the most of the world to come do the most here.
In practice, I think that there are a great many people at least in modern times who don't think this way. They've, according to Lewis' view (and I happen to agree with him at least in the sense that I think this is the only point of view in this area that has it doing any good), misunderstood the point of thinking on the next world. They think that it means that they shouldn't care at all for the things of this world because they are transitory and God is going to wipe it all out and bring it back in perfected form anyway so it doesn't matter. But it *does*, because we're not just spirit which is what the next world is concerned with but flesh and we have been given custody of the physical realm for a time. Not to neglect and abuse it but to work with and to show God that we understand what a gift we've been given.
Lewis seems to think that the neglect of the world is due to Christians ceasing to think of the next and perhaps is some cases this is so. But I maintain that there are too many who neglect this world because they think too much on the next.