With M. Teresa in view...

You must have often wondered why the enemy [God] does not make more use of his power to be sensibly present to human souls in any degree he chooses and at any moment. But you now see that the irresistible and the indisputable are the two weapons which the very nature of his scheme forbids him to use. Merely to over-ride a human will (as his felt presence in any but the faintest and most mitigated degree would certainly do) would be for him useless. He cannot ravish. He can only woo. For his ignoble idea is to eat the cake and have it; the creatures are to be one with him, but yet themselves; merely to cancel them, or assimilate them, will not serve.... Sooner or later he withdraws, if not in fact, at least from their conscious experience, all supports and incentives. He leaves the creature to stand up on its own legs--to carry out from the will alone duties which have lost all relish.... He cannot "tempt" to virtue as we do to vice. He wants them to learn to walk and must therefore take away his hand.... Our cause is never more in danger than when a human, no longer desiring, but still intending, to do our enemy's will, looks round upon a universe from which every trace of him seems to have vanished, and asks why he has been forsaken, and still obeys.*
-Uncle Screwtape
C.S. Lewis, The Screwtape Letters

*See also, Mother Teresa's Crisis of Faith in TIME