C. S. Lewis from Mere Christianity:
“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 that 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 French-man 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 realize 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 realized 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 forever. We must take it or leave it.”
From Mere Christianity