“So then it depends not on human will or exertion, but on God, who has mercy.” Romans 9:16 (ESV)
“But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.” John 1:12-13 (ESV).
Piper’s further, detailed argument for 9.16 as speaking of unconditional bestowal of divine mercy founders on both fundamental presupposition and its particulars. For the former, Piper assumes that the language of 9.16 is incompatible with God bestowing his mercy on a condition sovereignly determined by himself. But our exegesis has found this to be a false assumption. As for the particulars, appeals to 9.11-12 and Exod. 33.19 are contradicted by our exegesis of these texts as well as of 9.16, and the reader is directed to the relevant portions of the present volume. Curiously, Piper’s final main argument invokes Phil. 2.13 (because of the somewhat similar language of ‘willing’ (τὸ θέλειν) and ‘working’ (τὸ ἐνεργεῖν)) as somehow ruling out any condition for the bestowal of God’s mercy. But that text does not particularly talk about God’s mercy (except insofar as any blessing of God can be considered mercy) and it does not indicate anything about God’s bestowal of mercy, or any divine action, being unconditional. Piper seems to be overreaching here, and we conclude that Phil. 2.13 is largely irrelevant to Rom. 9.16 and the question of the conditionality of the mercy it mentions.
Piper, 154 n. 3, notes one further reference, cited by Sanday/Headlam as an analogy to 9.16 (though Piper mistakenly refers to 9.6): Jn 1.12-13. This reference actually works against Piper because the regenerating act of God there, performed by God alone, is presented as the divine response to human faith (cf. justification in Paul’s thought, which is performed by God alone in response to human faith). John 1.12 indicates that people become children of God by faith. That is, upon believing, God gives them the right to become something that they were not prior to believing – children of God. John 1.13 then clarifies that they become children of God not from human ancestry (that is the significance of ‘not of blood, nor of the desire of the flesh [which equates to sexual desire that might lead to procreation], nor of the will of a husband [who was thought to be in charge of sexual/procreative activity]’), but from God, describing their becoming children of God as being born of God. ‘Becoming children of God’ and ‘being born of God’ are parallel expressions referring to the same phenomenon (it would be special pleading, and a desperate expedient at that, to argue that becoming God’s child and being born of him are distinct in the Johannine context or that the text would allow that a person could be born of God and yet not be his child), so that God’s act of regenerating believers, making them his own children, is a response to their faith.
The parallel with Rom. 9.16 is significant and quite supportive of our exegesis. Both contexts make the point that elect status (which equates to sonship; cf. Rom. 9.8) is not bestowed by human ancestry, but by God, whose will is to choose as his own those who believe in Christ. Even if one were to deny that reference to θελήματος σαρκός or θελήματος ἀνδρός is to human ancestry specifically and insist that it refers to human willing in general, it would not make the divine action of regeneration any less a response to human faith and hence any less conditional on it. Nor would this be inconsistent with Jn 1.13’s attribution of the act of regeneration to God. The text indicates that God is the one who grants the right to become children of God and the one who regenerates. His doing so in response to faith is a matter of his discretion and would not somehow make the human choice to believe the source of regeneration instead of God any more than it makes it the source of justification. (Excerpt from footnote #153 on page 191 of Paul’s Use of the Old Testament in Romans 9.10-18: An Intertextual and Theological Exegesis, by Dr. Brian Abasciano, paragraph breaks added for easier reading)