Redefining the relationship between nature and grace was at the forefront of Reformation theology, and the reformers were keenly aware of the practical implications of their doctrinal formulations. The proper interpretation of Romans 9 was central to these debates. In his exegesis of verse 16, Martin Luther (1483-1546) teaches that human willing is only pleasing to God when empowered by him and undertaken according to his will. He warns his listeners to take care as they engage in related debates, counselling the need for wisdom and maturity rather than impetuous rushes of speculation.

Romans 9:16

It does not, therefore, depend on human desire or effort, but on God’s mercy. 

Commentary on Romans 9:16

It does not follow from this text that a person’s willing and running achieves nothing, but rather that it is not a matter of their own power. The work of God is not nothing. But a person’s willing and running is the work of God. For Paul is speaking here about willing and running according to God, that is, about the life of love and the righteousness of God. But any other kind of willing and running is nothing—no matter how intensely those who do not will and run in the way of God will and how vigorously they run. For these things are not of God and are not pleasing to him. . . .

Nevertheless I am warning you that anyone whose mind has not yet been purged should not rush into these speculations, lest they fall into the abyss of horror and hopelessness. Instead, they should first purge the eyes of their heart in their meditation on the wounds of Christ. I myself would not even read these things if the lectionary and necessity did not compel me to do so. For this is very strong wine and the most complete meal, solid food for those who are perfect, that is, the most excellent theology, of which the apostle says: “Among the mature we impart wisdom.” But I am a baby who needs milk, not solid food. Whoever is a child like me should do the same. The wounds of Christ, “the clefts of the rock,” are sufficiently safe for us.

Romans 9-16, ed. Philip D.W. Krey and Peter D.S. Krey, Reformation Commentary on Scripture NT vol. 8, 37.