I always enjoy J.I. Packer. I read this in his book Great Joy, and as I meditated on it, I thought of Colossians 1:29: "For this I toil, struggling with all his energy that he powerfully works within me."
Sanctification, we now see, is both a gift (that is one side: God working in us to renew and transform us) and a task (the task of obedience, righteousness, and pleasing God). And we must never so stress either of the two sides that we lose sight of the other. Think only of the task, and you will become a self-reliant legalist seeking to achieve righteousness in your own strength. You will not make any headway at all. Think only of the work of God in your life, and the chances are that Satan will trick you into not making the necessary effort and not maintaining the discipline of righteousness so that, in fact, even as you rejoice in the work of God in your life you will be dishonoring it by your slackness. Hold both sides of the matter together in your mind, if you want your living to be right.