But why does Jacob say: “Break or get grain for us, that we may live and not die”? Why does he not trust God? Why does he fear death, since he has a divine promise of preservation and protection? Besides, he has the experience by which he has so far been aware of and felt the help and guidance of God in the foreign land of Canaan together with so many sons, so many grandsons, and a very large household. Why does he tremble here and not trust the promises which he undoubtedly bore in mind and carefully taught in the church in his home? “Do not be broken by trials,” he says. “God has promised us the land and in it the food that is needed.” Where is your faith now, Jacob? Where is the promise?
I reply that we are ordered to believe, and to trust in the goodness of God, but not to tempt God. For we cannot live our life according to the rule which He Himself has, but we must live as opportunities and the times decide. Yet faith and hope must be preserved. Therefore Jacob does not say: “Remain. Wait. The Lord has the power to send bread from heaven. Perhaps He will cause grain to rain in this land and nourish us.” This is not what the promise means. Although there is no doubt that God is able and willing to nourish and defend you, you must not avoid the opportunities offered for help and protection. You can avail yourself of these without sin. Otherwise you will be tempting God.
Luther’s works, vol. 7: Lectures on Genesis: Chapters 38-44 p 219