Here is another way of putting the two sides of the truth. On the one hand we must never imagine that our own unaided efforts can be relied on to carry us even through the next twenty-four hours as ‘decent’ people. If He does not support us, not one of us is safe from some gross sin. On the other hand, no possible degree of holiness or heroism which has ever been recorded of the greatest saints is beyond what He is determined to produce in every one of us in the end. The job will not be completed in this life; but He means to get us as far as possible before death.
That is why we must not be surprised if we are in for a rough time. When a man turns to Christ and seems to be getting on pretty well (in the sense that some of his bad habits are now corrected) he often feels that it would now be natural if things went fairly smoothly. When troubles come along—illnesses, money troubles, new kinds of temptation—he is disappointed. These things, he feels, might have been necessary to rouse him and make him repent in his bad old days; but why now? Because God is forcing him on, or up, to a higher level: putting him into situations where he will have to be very much braver, or more patient, or more loving, than he ever dreamed of being before. It seems to us all unnecessary: but that is because we have not yet had the slightest notion of the tremendous thing He means to make of us.
Quotes from Mere Christianity, Part 81
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 204-205.