Sanctification is a work of the Triune God, that is ascribed more particularly to the Holy Spirit in Scripture. It is particularly important in our day, with its emphasis on the necessity of approaching the study of theology anthropologically and its one-sided call to service in the kingdom of God, to stress the fact that God, and not man, is the author of sanctification. Especially in view of the Activism that is such a characteristic feature of American religious life, and which glorifies the work of man rather than the grace of God, it is necessary to stress the fact over and over again that sanctification is the fruit of justification, that the former is simply impossible without the latter, and that both are the fruits of the grace of God in the redemption of sinners. Though man is privileged to cooperate with the Spirit of God, he can do this only in virtue of the strength which the Spirit imparts to him from day to day. The spiritual development of man is not a human achievement, but a work of divine grace. Man deserves no credit whatsoever for that which he contributes to it instrumentally… A man may boast of great moral improvement, and yet be an utter stranger to sanctification.
Louis Berkhof, Systematic Theology, pages 532, 535.