I don't understand the reasoning to circumcise for religion. Why would a god require mutilation of his own creation?
Well, lots of people don't believe a God, or in this instance Yahweh, required it at all. Much of the scholarly theological work of the past 20 years or so has come to the conclusion that circumcision was never prescribed by God, and that it was actually pushed by the (now discredited and gone) priests of Judaism as a way to rally and control the Hebrews when the priests were starting to lose their power. It was a practice well-known at the time (Babylonian period) but not generally practiced by the Hebrews, and the priests wove it clumsily back through their history, first orally and later in written form. It is not in dispute that the first 4 known iterations of the pentateuch (starting with what many now call the Book of J) make no reference to infant circumcision; the fifth version - the basis for our current Old Testament - has a somewhat awkward chapter 17 that looks to be little more than an elaboration of Chapter 15, but with circumcision thrown in. Scholars refer to this latter version of the pentateuch (or Torah) as the Book of P, a reference to the priests who apparently re-wrote it.
The procedure the priests advocated wasn't awfully controversial; it involved cutting just the overhanging tip of the newborn's foreskin and even a father could do it more or less successfully with a knife. This seems to have been the course for about 300 hundred years until the Hellenic period, when it was highly out of fashion to be cut, and some younger Jewish men sought to reverse their half-circumcisions through stretching (epispasm). The rabbis became alarmed -- the corrupt priests were long-gone -- and ordered up a whole new type of circumcision contemplated nowhere in Genesis. This one came in 3 parts: cutting the skin (milah), tearing the binding synechia between the glans and foreskin of newborns (periah) and severing any skin that might touch the glans, and then sucking the wound for 30 seconds (metzitza). This rather severe new modification to circumcision also required the introduction of an entirely new profession also not mentioned anywhere in the Bible: the mohel, or skilled circumcision practitioner, since this newly prescribed surgery was unsafe in the hands of most new dads. The party thrown today (bris) so the community of friends and relatives can witness the newborn's foreskin being cut off also has no biblical or divine basis; it seems to have evolved as a way to prevent parents from backing out of the more extensive circumcision through peer pressure.
When examined critically, it seems pretty obvious that circumcision is man-made, not divinely ordained. Even if one were utterly convinced of the facts of Genesis 17 (a stretch, since even moderate scholars agree that Abraham was mythical), there is no evidence that the radical circumcision seen today had anything to do with Old Testament instructions or practice. It emerged as an extreme clergical response to perceived vanity in a culture that idolized male nudity and integrity.
Fortunately, the practice remained relatively contained to specific religious groups until some quack US and English doctors revisited the procedure -- but this time on toddlers and young boys -- in the late 1800's as a possible remedy for laziness, palsy and masturbation. By the early 20th centruy it became "medicalized" as a maternity procedure in most English-speaking countries with varying prevalence. The fad expanded in some countries and declined in others, with the US being nearly alone now in circumcising most male newborns for non-religious reasons. Worldwide, approximately 3 out of every 100 boys under the age of five is circumcised, and the US accounts for 2 of the 3. (The percentage goes up by puberty with the inclusion of Muslims, who normally wait.) The trend today is decidedly downward, as health benefits are nearly impossible to ascertain or prove and the collective cost cannot be justified. Also, new research into the function of the foreskin and early-childhood development of the penis tend to contraindicate circumcision's desirability or effectiveness.
This has nothing to do with post-puberty circumcision, which has its own considerations.
