The promise of the Return

We must never speak to simple, excitable people about "the Day" without emphasizing again and again the utter impossibility of prediction. We must try to show them that that impossibility is an essential part of the doctrine. If you do not believe our Lord's words, why do you believe in his return at all? And if you do believe them must you not put away from you, utterly and forever, any hope of dating that return? His teaching on the subject quite clearly consisted of three propositions, (1) That he will certainly return. (2) That we cannot possibly find out when. (3) And that therefore we must always be ready for him.

Note the therefore. Precisely because we cannot predict the moment, we must be ready at all moments. Our Lord repeated this practical conclusion again and again; as if the promise of the Return had been made for the sake of this conclusion alone. Watch, watch, is the burden of his advice. I shall come like a thief. You will not, I most solemnly assure you you will not, see me approaching. If the householder had known at what time the burglar would arrive, he would have been ready for him. If the servant had known when his absent employer would come home, he would not have been found drunk in the kitchen. But they didn't. Nor will you. Therefore you must be ready at all times. The point is surely simple enough. The schoolboy does not know which part of his Virgil lesson he will be made to translate: that is why he must be prepared to translate any passage. The sentry does not know at what time an enemy will attack, or an officer inspect, his post: that is why he must keep awake all the time. The Return is wholly unpredictable. There will be wars and rumours of wars and all kinds of catastrophes, as there always are. Things will be, in that sense, normal, the hour before the heavens roll up like a scroll. You cannot guess it. If you could, one chief purpose for which it was foretold would be frustrated. And God's purposes are not so easily frustrated as that.

 -- C.S. Lewis, The World's Last Night (1960)