This is not a promotion or an endorsement of Piper as a true believer. But Piper is quite correct in his assessment of the carnal (unregenerate) foolishness of Forster and Marston.
John Piper, from his book The Justification of God:
“Forster and Marston are pressed by the weight of this argument to give a unique and wholly improbable interpretation of Paul’s response in 9:20. They argue in the following way:
‘Yet Paul’s angry reply: ‘Nay, but O man, who are you that replies against God?’ itself demonstrates the stupidity of such a misrepresentation. How could the man reply against God if, as he supposed, he could not resist God’s will? Therefore, Paul says, ‘Nay, rather, you yourself are resisting it now!”
What is evident from this remarkable construction is that the objector of Rom 9:19 understands Paul much better that Forster and Marston do. For when they ask rhetorically, How could a person reply against God without resisting God’s will? the objector could very simply answer, Because my reply is a fulfillment of the hardening will of God. In other words, Forster and Marston have not grasped as well as the objector that Paul sees Pharaoh’s ‘resistance’ of God’s commands as a fulfillment of God’s hardening decree. They view God’s will in such a way that, if God says to Pharaoh, ‘Let my people go,’ and Pharaoh says, ‘No,’ then we have proof that men can and do resist God’s will, and the objector has erected a bogus difficulty that does not really exist in Paul’s theology.
Against this interpretation stands first, as we have seen, the fact that Paul, unlike Morison and Forster and Marston, does not give the slightest trace of disagreement with the objector’s interpretation of Rom 9:18. On the contrary, his response in 9:20-23 not only affirms but heightens (if that is possible) the absoluteness of God’s sovereign will in disposing all things.
Secondly, the conclusion of our own exegesis in Chapter Nine, if it is correct, confirms that objector’s interpretation of Rom 9:18, not Forster and Marston’s. Accordingly, when Paul rebukes the objector, he accuses him not of misconstruing Rom 9:18 but of presuming to question the rightness of God’s dealing (Barret, Romans, 187; Schlatter, Gerechtigkeit, 303; Gifford, Romans, 172; Meyer, Romans, II, 143). ‘It is precisely because the objection has the character of insolence rather than anguish that Paul responds so sharply’ (Lagrange, Romains, 236).
To be sure, Pharaoh said ‘No’ to God’s command that he send the Israelites into the wilderness. This can reasonably be called ‘resisting’ God (cf. Acts 7:51). But this is so obvious to everyone that it is utterly implausible that the objector would be affirming that no one has ever resisted God in this sense. Everyone has. But Paul’s point was that even this resistance is in one sense willed ([Greek word cited here by Piper–CD] 9:18) by God as hardness. The objector sees clearly that Paul is saying: God wills that Pharaoh resist God’s own commands ” (John Piper, pp. 190-191).
 Even Forster and Marston would have to admit this in some cases because they think that after the fifth plague God gave Pharaoh ‘supernatural strength to continue with his evil path of rebellion’ (God’s Strategy, 73). In other words, it was in some sense God’s will that for four more plagues Pharaoh not let the people of Israel go. Nevertheless, even after God had willed not to let Israel go, ‘The Lord said to Moses: ‘Go to Pharaoh and say to him, ‘Thus says the Lord, Let my people go!’ (Ex 8:1). So even in their scheme, Forster and Marston have to distinguish between God’s ‘will of command’ and his ‘will of decree.'”