From The Screwtape Letters - by C.S. Lewis - excerpted from letter 15.
Letters from one demon to another (i.e. the 'Enemy' is God):
"The humans live in time but our Enemy destines them to eternity. He therefore, I believe, wants them to attend chiefly to two things, to eternity itself, and to that point of time which they call the Present. For the Present is the point at which time touches eternity.
"Of the present moment, and of it only, humans have an experience analogous to the experience which our Enemy has of reality as a whole; in it alone freedom and actuality are offered them.
"He would therefore have them continually concerned either with eternity (which means being concerned with Him) or with the Present—either meditating on their eternal union with, or separation from, Himself, or else obeying the present voice of conscience, bearing the present cross, receiving the present grace, giving thanks for the present pleasure.
"Our business is to get them away from the eternal, and from the Present.
"With this in view, we sometimes tempt a human (say a widow or a scholar) to live in the Past. But this is of limited value, for they have some real knowledge of the past and it has a determinate nature and, to that extent, resembles eternity. It is far better to make them live in the Future.
"Biological necessity makes all their passions point in that direction already, so that thought about the Future inflames hope and fear. Also, it is unknown to them, so that in making them think about it we make them think of unrealities.
"In a word, the Future is, of all things, the thing least like eternity. It is the most completely temporal part of time—for the Past is frozen and no longer flows, and the Present is all lit up with eternal rays. (...)
"To be sure, the Enemy wants men to think of the Future too—just so much as is necessary for now planning the acts of justice or charity which will probably be their duty tomorrow. The duty of planning the morrow's work is today's duty; though its material is borrowed from the future, the duty, like all duties, is in the Present.
"This is not straw splitting. He does not want men to give the Future their hearts, to place their treasure in it.
"His ideal is a man who, having worked all day for the good of posterity (if that is his vocation), washes his mind of the whole subject, commits the issue to Heaven, and returns at once to the patience or gratitude demanded by the moment that is passing over him.
"But we want a man hag-ridden by the Future—haunted by visions of an imminent heaven or hell upon earth—ready to break the Enemy's commands in the present if by so doing we make him think he can attain the one or avert the other—dependent for his faith on the success or failure of schemes whose end he will not live to see.
"We want a whole race perpetually in pursuit of the rainbow's end, never honest, nor kind, nor happy now, but always using as mere fuel wherewith to heap the altar of the future every real gift which is offered them in the Present."