Why does Paul appeal to the witness of the Holy Spirit in Romans 9:1?

Romans 8:1–39 had concluded with the apostle expressing the firm conviction that nothing can separate us from the love of God (Romans 8:38–39).  This very personal exclamation is immediately followed by another, albeit with a different tone (Romans 9:1–5). Paul begins Romans 9:1–33 with an abrupt declaration of uprightness: I am speaking the truth in Christ—I am not lying; my conscience bears me witness in the Holy Spirit (Romans 9:1). This declaration immediately grabs the reader’s attention. It generates the anticipation that something unexpected is to follow. And this is indeed precisely what happens. It is as though Paul suddenly descends from the high he had just been on, for Romans 8:1–39 most certainly concludes on a high note: If God is for us, who can be against us?…nor height not depth…will be able to separate us from the love of God in Christ Jesus our Lord (Romans 8:31b, Romans 8:39). Yet suddenly the apostle now falls into a precipitous depth: I have great sorrow and unceasing anguish in my heart (Romans 9:2). It seems as though he first gathered courage needed to be able to actually dare to look into the depth. He knows that nothing will separate him from the love of God, yet despite this his heart is numb with sorrow—and that, he emphasises, is no lie.¹