"The church that approves of homosexual relations has by that act ceased to be a true church. Wolfhart Pannenberg"
Piper takes a traditionally selective and apologetic movement through the scriptures to support his claims, starting with Paul's category list of sins in 1 Cor. 6:9-10. Here is Piper's citation of it, including the translation he uses and his italics for emphasis: “Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God."
The subterfuge only begins here. For one, Piper has stuck with his own specially selected translation of the infamous Greek ἀρσενοκοίτης. I say infamous because it has been the subject of numerous journal articles, scholarly monographs, and debates over the years for the simple reason that it's a hapax. Hapax in Greek means "once" and the full term is hapax legomenon, that is, "spoken/read once," a word appearing only one time. Aside from its repetition by the author of 1 Timothy (almost certainly not Paul), this word appears nowhere else in the vast corpus of Greek literature prior to here.
The most convincing case I have seen is that Paul made the word up based on the Greek Septuagint (the imperfect Greek translation of the Jewish scriptures by rabbis in Alexandria sometime in the second century BCE) version of Leviticus 18 and 20, in which ἄρσην (male) and κοίτη (bed, or a lying) appear in rough proximity. But regardless, the meaning of this word (or I should say, the range of meanings, since no ancient word has a perfect one-to-one translation into modern English) is far from certain.
Not only that, but Paul elsewhere in Romans 1 (either speaking for himself or echoing a rhetorical spokesperson) appears to connect men burning with lust for one another with a sort of natural theology -- that is, he considers it decidedly unnatural. Why is this relevant? Because there is a huge debate going on in classical studies and Biblical studies as to how the ancients viewed sexuality and in particular, homosexuality. Did they view is as a matter of "nature"? Indeed, the work at the center of my dissertation, Longus' Daphnis and Chloe, refers to a particularly unflattering character as a "pederast by nature." But that only further complicates the issue with Paul's ἀρσενοκοίτης, for there were existing and available words for active (pederast or an erastes) and passive (cindaedus or eromenos) "homosexuals," but Paul chose to make up some other word.
Further, did you notice how Piper uses the translation "men who engage in homosexual acts"? Quite a long translation for one word (not that that's uncommon). And since many have noted Paul's preoccupation with "nature" (φύσις) in Romans 1, it's at least up for debate whether Paul or others would have separated "homosexuality" by nature from "homosexual acts." Indeed, this is the crux of the controversy mentioned above. But by translating it "homosexual acts," Piper can elide all these concerns and stay within the "love the sinner/hate the sin" mantra, by disavowing the older "homosexuals" translation and instead focusing on "homosexual acts." It's a deft and subtle movement, but he knows what he's doing.
But it's only more puzzling that Piper then connects this statement by Pannenberg to Bonhoeffer, in an obvious attempt to gird his own Christian Zionism. To suggest that the New Testament is a set of texts that unambiguously expresses "standing with the Jews" is to engage in the highest level of intellectual dishonesty. The Gospel of John even has Jesus calling the Jews the children of the devil, and even Paul's oft-quoted "there is neither Jew nor Greek" in Galatians 3:28 has been viewed by Jews as anti-Jewish: for it subsumes or eradicates them from existence. Unfortunately, much of the New Testament exhibits this sort of anti-Jewish polemic.
But then, Piper is not really trying to make an intellectually honest examination of sexuality and the Bible (yes, I'm aware he has published in scholarly journals: that does not make his approach in this case intellectually honest). Rather, he is seeking to prop up his predetermined views on gender and sexuality by simplistically and selectively proof-texting them: an unfortunate technique nevertheless as old as Paul and Matthew. And for good measure, he's sprinkled in some big German names!
Piper and his lot start with a fallacy: that the Bible is 100 percent consistent in its teachings, that it is one book, and that a body must cohere to it to the letter (presumably even its injunctions that slaves obey their masters, no matter how cruel or if they beat them) to remain a "true Church." This is a modern-day notion (dare I say heresy?) that elevates the Bible to divine status. It is all the more despicable when it is done in order to marginalize and oppress a given group, even if one considers that group outside of the church: when scriptures are taken at face value, with little to no scrutiny and no appreciation for their immediate and ancient context, and used as a weapon to often violent ends toward people considered outside of the accepted circle.
As indeed the church was still doing to the Jews in Bonhoeffer's time, using the same New Testament (and indeed, the same selective methodology) Piper is using today.