C. S. Lewis, in Mere Christianity:
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.
It does not mean that we are to leave the present world as it is.
If you read history you will find that the Christians who did most for the present world were just those who thought most of the next.
The Apostles themselves, who set on foot the conversion of the Roman Empire, the great men who built up the Middle Ages, the English Evangelicals who abolished the Slave Trade, all left their mark on Earth, precisely because their minds were occupied with Heaven.
It is since Christians have largely ceased to think of the other world that they have become so ineffective in this.
Aim at Heaven and you will get earth ‘thrown in’: aim at earth and you will get neither.”
Yes, I know. It is possible to be so heavenly minded that we are of no earthly use. My problem is: I’ve never met one of those people. And I suspect, if I met one, the problem would not be that his mind is full of the glories of heaven, but that his mind is empty and his mouth is full of platitudes.
I suspect that for every professing believer who is useless in this world because of other-worldliness, there are a hundred who are useless because of this-worldliness.