In Mere Christianity, C. S. Lewis writes:
Enemy-occupied territory — that is what this world is. Christianity is the story of how the rightful king has landed, you might say landed in disguise, and is calling us all to take part in a great campaign of sabotage. When you go to church you are really listening — in to the secret wireless from our friends: that is why the enemy is so anxious to prevent us from going. He does it by playing on our conceit and laziness and intellectual snobbery. I know someone will ask me, “Do you really mean, at this time of day, to reintroduce our old friend the devil — hoofs and horns and all?” Well, what the time of day has to do with it I do not know. And I am not particular about the hoofs and horns. But in other respects my answer is “Yes, I do.” I do not claim to know anything about his personal appearance. If anybody really wants to know him better I would say to that person, “Don’t worry. If you really want to, you will Whether you’ll like it when you do is another question.” 
Christianity is the story of how the rightful king has landed, you might say landed in disguise, and is calling us all to take part in a great campaign of sabotage.
He goes on to address a potential objection:
Why is God landing in this enemy-occupied world in disguise and starting a sort of secret society to undermine the devil? Why is He not landing in force, invading it? Is it dial He is not strong enough? Well, Christians think He is going to land in force; we do not know when. But we can guess why He is delaying. He wants to give us the chance of joining His side freely. I do not suppose you and I would have thought much of a Frenchman who waited till the Allies were marching into Germany and then announced he was on our side. God will invade. But I wonder whether people who ask God to interfere openly and directly in our world quite realise what it will be like when He does. When that happens, it is the end of the world. When the author walks on to the stage the play is over. God is going to invade, all right: but what is the good of saying you are on His side then, when you see the whole natural universe melting away like a dream and something else — something it never entered your head to conceive — comes crashing in; something so beautiful to some of us and so terrible to others that none of us will have any choice left? For this time it will be God without disguise; something so overwhelming that it will strike either irresistible love or irresistible horror into every creature. It will be too late then to choose your side. There is no use saying you choose to lie down when it has become impossible to stand up. That will not be the time for choosing: it will be the time when we discover which side we really have chosen, whether we realised it before or not. Now, today, this moment, is our chance to choose the right side. God is holding back to give us that chance. It will not last for ever. We must take it or leave it. 
- Lewis, C. S. Mere Christianity (New York: Book-of-the-Month Club, Inc., 1993), 36.
- Ibid, 50-51.