God could easily give you grain and fruit without your plowing and planting. But He does not want to do so. Neither does He want your plowing and planting alone to give you grain and fruit; but you are to plow and plant and then ask His blessing and pray: “Now let God take over; now grant grain and fruit, dear Lord! Our plowing and planting will not do it. It is Thy gift.”...
What else is all our work to God—whether in the fields, in the garden, in the city, in the house, in war, or in government—but just such a child’s performance, by which He wants to give His gifts in the fields, at home, and everywhere else? These are the masks of God, behind which He wants to remain concealed and do all things. Had Gideon done nothing but take the field against Midian, the Midianites would not have been beaten; and God could certainly have beaten them without Gideon. He could give children without using men and women. But He does not want to do this. Instead, He joins man and woman so that it appears to be the work of man and woman, and yet He does it under the cover of such masks. We have the saying: “God gives every good thing, but not just by waving a wand.” God gives all good gifts; but you must lend a hand and take the bull by the horns; that is, you must work and thus give God good cause and a mask.
Martin Luther (LW 14:114-115)