A farmer plows his field, sows the seed, and fertilizes and cultivates - all the while knowing that in the final analysis he is utterly dependent on forces outside of himself. He knows he cannot cause the seed to germinate, nor can he produce the rain and sunshine for growing and harvesting the crop. For a successful harvest, he is dependent on these things from God.
Yet the farmer knows that unless he diligently pursues his responsibilities to plow, plant, fertilize, and cultivate, he cannot expect a harvest at the end of the season. In a sense he is in a partnership with God, and he will reap its benefits only when he has fulfilled his own responsibilities.
Farming is a joint venture between God and the farmer. The farmer cannot do what God must do, and God will not do what the farmer should do.
We can say just as accurately that the pursuit of holiness is a joint venture between God and the Christian. No one can attain any degree of holiness without God working in his life, but just as surely no one will attain it without effort on his own part. God has made it possible for us to walk in holiness. But He has given to us the responsibility of doing the walking; He does not do that for us.
(from pp. 11-12 of the Preface to the 25th Anniversary Edition of The Pursuit of Holiness by Jerry Bridges, copyright 1978, 1996, 2003)