The practical upshot is this. On the one hand, God’s demand for perfection need not discourage you in the least in your present attempts to be good, or even in your present failures. Each time you fall He will pick you up again. And He knows perfectly well that your own efforts are never going to bring you anywhere near perfection. On the other hand, you must realise from the outset that the goal towards which He is beginning to guide you is absolute perfection; and no power in the whole universe, except you yourself, can prevent Him from taking you to that goal. That is what you are in for. And it is very important to realise that. If we do not, then we are very likely to start pulling back and resisting Him after a certain point. I think that many of us, when Christ has enabled us to overcome one or two sins that were an obvious nuisance, are inclined to feel (though we do not put it into words) that we are now good enough. He has done all we wanted Him to do, and we should be obliged if He would now leave us alone. As we say ‘I never expected to be a saint, I only wanted to be a decent ordinary chap.’ And we imagine when we say this that we are being humble.
But this is the fatal mistake. Of course we never wanted, and never asked, to be made into the sort of creatures He is going to make us into. But the question is not what we intended ourselves to be, but what He intended us to be when He made us.
Quotes from Mere Christianity, Part 80
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 201-202.