N.T. Wright gives, I think, an important hermeneutical tool for Romans 9. He says,
if there is complete disjunction between God’s justice and everybody else’s, it would be better not to use the term at all. (Commentary on Romans, p. 639).
Having previously held to the hermeneutic “God can do whatever he wants” I no longer think it is adequate for this passage. In other words, it is not accurate to interpret Paul as saying that God hardens certain people at random – apart from anything in their nature – just because he’s God. Wright’s comment reminded me of something Anselm said in Cur Deus Homo,
the argument that, ‘If it is God’s will to tell a lie, it is just to tell a lie’, is a non sequitur … Unless, that is, we adopt an interpretation of the kind used when we say with reference to two impossibles, ‘If this thing is so, then that thing is so’, when neither ‘this’ nor ‘that’ is the case; for instance, if one were to say, ‘If water is dry, then fire is wet’, given that neither is true. It is therefore only true to make the statement, ‘If it is God’s will, then it is just’, about things which it is not unfitting for God to wish.