“If there be so certain and glorious a rest for the saints, why is there no more industrious seeking after it? One would think, if a man did but once hear of such unspeakable glory to be obtained, and believed what he heard to be true, he should be transported with the vehemency of his desire after it, and should almost forget to eat and drink, and should care for nothing else, and speak of and inquire after nothing else, but how to get this treasure. And yet people who hear of it daily, and profess to believe it as a fundamental article of their faith, do as little mind it, or labour for it, as if they had never heard of any such thing, or did not believe one word they hear.” Richard Baxter, 1649
“Since, then, you have been raised with Christ, set your hearts on things above, where Christ is seated at the right hand of God.” The Apostle Paul, Colossians 3:1
“Let temporal things serve your use, but the eternal be the object of your desire.” Thomas a Kempis
“It is since Christians have largely ceased to think of the other world that they have become so ineffective in this one.” C. S. Lewis
“It has been cited as a flaw in Christianity that it is more concerned with the world to come than with the world that now is, and some have been fluttering about trying to defend the faith of Christ against this accusation. Both the attack and the defense are wasted. No one who knows what the New Testament is about will worry over the charge that Christianity is other-worldly. Of course it is, and that is precisely where its power lies.
“Let no one apologize for the powerful emphasis Christianity lays upon the doctrine of the world to come. Right there lies its immense superiority to everything else within the whole sphere of human thought or experience. When Christ arose from death and ascended into heaven He established forever three important facts, namely, that this world has been condemned to ultimate dissolution, that the human spirit persists beyond the grave and that there is indeed a world to come.
“The church is constantly being tempted to accept this world as her home, and sometimes she has listened to those who would woo her away and use her for their own ends. But if she is wise she will consider that she stands in the valley between the mountain peaks of eternity past and eternity to come. The past is gone forever and the present is passing as swift as the shadow on the sun dial of Ahaz. Even if the earth should continue a million years not one of us could stay to enjoy it. We do well to think of the long tomorrow.” A. W. Tozer
For more information on this subject, see Randy Alcorn's book Heaven.