When looking at the cultural norms of the ancient world, we often see that what one culture believed to be normal, they often deemed to be natural. In other words, what is natural is culturally conditioned and changes from culture to culture. This conception of nature has been explored by John J. Winkler in The Constraints of Desire, where he remarks that when we see “nature” in ancient Greek texts, we should actually read “culture.” “Indeed what ‘natural’ means in many such contexts is precisely ‘conventional and proper.’ The word ‘unnatural’ in contexts of human behavior quite regularly means ‘seriously unconventional’ and is used like a Thin Ice sign to mark off territory where it is dangerous to venture.” In a similar vein, Philo also offers comment on ways in which customs become a part of what is considered natural over the course of time. “They are under the sway of a very ancient custom, which through long familiarity has won its way to the standing of nature (γενόμενον εἰς φύσιν ἐκνενίκηκεν)” (Special Laws 21.109).
Exploring the topic of foreskin and circumcision in the ancient world reveals the same understanding of nature. In various medical texts and handbooks we often see culturally situated penile aesthetics being described as natural. In his monumental work, Greek Homosexuality, K. J. Dover comments on the near obsession ancient Greeks had with foreskin when it came to artistic depictions of the penis. Penises were depicted as being petit and having long tapered foreskins. While the medical texts may not be as descriptive as the artwork, they do attest to Greek (and Roman) desire to have a specific look to the foreskin, which they deemed to be “natural.”
In his Gynecology, Soranus describes the process through which a midwife can mold an infant into its “natural (κατὰ φύσιν) shape” (Gynecology 2.9.14). Concerning the foreskin he writes, “If the infant is male and it looks as though it has no foreskin, she should gently draw the tip of the foreskin forward or even hold it together with a strand of wool to fasten it. For if gradually stretched and continuously drawn forward it easily stretches and assumes its normal length, covers the glans and becomes accustomed to keep the natural good shape (τὴν φυσικἠν εὐμορφίαν)” (Gynecology 2.16.34). Similar procedures are outlined by Celsus and Galen to elongate or reconstruct a deficient foreskin. On the insufficient foreskin, Galen writes about “a departure in terms of magnitude from what accords with nature (κατὰ φύσιν)” (Method of Medicine 14.16). From this cursory overview, it seems evident that in the ancient Greco-Roman mind having a foreskin is natural and desirable.
When we take this knowledge in hand and begin to approach the Apostle Paul’s thoughts on circumcision, what do we find? In an often poorly translated section of Romans 2, Paul refers to those, presumably gentiles, who are “from nature foreskinned (ἐκ φύσεως ἀκροβυστία)” (Rom 2:27). While those of us in the modern world would understand all male infants to be foreskinned by nature, from this text it seems that in Paul’s mind only gentiles were naturally foreskinned. The corollary of this would be that Jews are by nature circumcised. In Galatians 2:15, Paul comments that “We are by nature Jews (ἡμεῖς φύσει Ἰουδαῖοι) and not gentile sinners.” Given that the key issue of Galatians pertains to gentile Judaizing (specifically undergoing circumcision), it is probable that this text corroborates with the above reading of Romans 2:27, that is, Paul, like his Greco-Roman contemporaries, had a culturally situated view of what was natural. Paul understood that Jews were by nature circumcised and gentiles were by nature foreskinned.
How does this impact the way we read Paul on circumcision? I would argue that for Paul, gentiles should remain in their natural state as foreskinned and Jews should also remain in their natural state as circumcised (à la 1 Cor 7:18). These were ethnic identities that he acknowledges and upholds. These categorical differences would naturally remain in place, even after the Messiah came. As we see in Romans 11:24, naturally wild branches (φύσιν ἀγριελαίου) are grafted, contrary to nature (παρὰ φύσιν), into a cultivated tree. While this is only one aspect of Paul’s thought regarding circumcision, I find it to be quite illuminating in regard to how we read him when it comes to Gentiles and Jews and the issue of circumcision. The above findings from Greco-Roman texts can also inform us when we seek to understand Paul’s gentile audiences and their opinions on circumcision, but that’s a subject for another essay.
So to respond to the question posed in the title, for Paul, I think his answer would be, “It depends.”
 John J. Winkler, The Constraints of Desire: The Anthropology of Sex and Gender in Ancient Greece (London: Routledge, 1990), 17.
 K.J. Dover, Greek Homosexuality (Cambridge, MA: Harvard University Press, 1989), 125-31.
 Most modern translations read, “physically uncircumcised.” In my understanding, ἀκροβυστία is best translated as “foreskin.”
 For another culturally situated instance of what is natural in Paul, see 1 Cor 11:14.