How can cutting skin do all that. Ok here goes. The amount of skin removed is one third to one half the total skin on the penis, and also contains more nerve endings than any other place on the penis, a great majority of this in the frenulum (which they used to remove completely, but now leave a portion behind, you know that ultra sensitive spot you guys love during oral sex? Imagine enjoying more of that). You lose more than half of the sensitivity potential -nerve endings, in removing it.. It's impossible to know at birth how much of that skin will be used as the penis grows in puberty. The male penis reaches it max length in the late teens early twenties. If they remove too much it can cause erectile problems, the skin can be so tight the penis curves when erect or has pubic hair on the shaft. The foreskin is also adhered to the glands at birth to prevent infections. It naturally separates as the child ages and completes around puberty, when the penile growth spurt starts. When circumcised this skin is forcibly ripped from the sensitive and very fragile glands underneath. This skin now has no protection and can suffer keritinization, which is a hardening and toughening of the inner skin which now has to function in a protective manner and not in the sensitive manner it was intended. Nerve endings die off. The skin dries up when it was biologically created to remain moist. Such moisture (smegma in hygienic amounts) contains human immunoglobin E (or is it A I can never remember), which is an antibody found generally only in breastmilk, and kills infection. Smegma (again in hygienic amounts) is the males version of discharge, and serves to keep the penis clean. This moisture also serves to help keep the female partner lubricated, and enhance male pleasure. The rubbing of the glands against the foreskin is a large part of the male orgasmic response. Coincidentally it also changes the stroke technique used in males to something more friendly to both sexes.
Because there is so much blood flow in the penis, even the infant penis, and the amount of skin is so great, the loss of blood can be high. Some babies can go into shock and die from either the pain, or the blood loss. It's only been in the last 20 or so years that they finally caught on that infants can feel pain and thus offer pain relief for the procedure. Unfortunately, it's never going to be complete, nor does it handle the recovery pain as acidic urine and feces and clothing rubs against a sensitive gland that is now exposed when it was meant to be protected.
Circumcision these days has as such gone to a more looser cut to reduce side effects, which ironically gives the look of half circumcised without the natural protections but with many more complications instead of fewer. The result is some foreskin remaining, the idea of which is to help prevent the erection problems of yesteryears. The downside is that the foreskin attempts to do what it was meant to do at this age and re-adhere to the glands, causing adhesions, which most doctors ignorantly treat by separating, ripping over and over again and building up scar tissue which further damages the skin and it's sensitivity.
Did you know the average uncut penis is half an inch larger than the circumcised? Possibly because it was allowed to grow to it's potential. Some studies are showing the male sexual problems of the mature crowd may be related to the aging effects of the damages caused by circumcision, which is why getting cut in your twenties will not give you an accurate portrayal of both sides. You need to tack on 20 years of the skin toughening up for it to even start to compare.
Now that even disregards the ethics of permanently altering another persons sexual organs without their permission, without complete pain control (they are too young for general anesthesia), and without regard to future sexual health.
Christians follow the New Testament, and in that case Jesus not only died for your sins, he was circumcised as well. I don't have the quote for you, but yes it's clear as day. And even in the old testament many theologians interpret it as 'cutting' not removing as is current common practice.