Get Earth "Thrown In"

I happened to come across this quote that Pastor Tommy Lee recently posted:

“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.”
~ C.S. Lewis