Someone else did the work on them (although it's possible to do it to themselves).
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First of all, while it may be possible for someone to perform that surgery upon themselves, there's no record in scripture that anyone ever did. Circumcision is always spoken of in scripture as a procedure that one person performed upon another (Gen 17:23-27; 21:4; Ex 4:25-26; 12:44, 48 ; Jos 5:2-3). And this makes good sense because it was a surgical procedure and quite painful before man invented anesthesia -- even after the 3rd day of surgery (Gen 34:24-25).
The second thing we learn about circumcision is that with the exception of any first generation (adult) male who desired to enter into a covenant relationship with YHWH through the divinely prescribed circumcision ritual, the future generation of males were required to be circumcised 8 days after their birth. This requirement was even embedded into the Law of Moses (Lev 12:3). What this means, then, is that in the vast majority of cases a circumcised male not only had the procedure performed on him but it was performed without the male's consent! In other words, the males' "free will" was circumvented! (Wow! Imagine that! Look at how many millions of males had God's will "forced" upon them -- even to this day!)
The third aspect to this religious ritual is what the "cutting off" of a male's foreskin symbolized. Without getting into the medical weeds, let's just say that this was part of a male's anatomy that was for all practical intent and purposes was useless. It would not be a stretch to say that that for this reason, the foreskin was already "dead" and needed to be separated from that of which it was an integral part so that whole would become clean. The removal of this "dead" foreskin was liekly a cleansing ritual of sorts by which males entered into a coveanat relationship with God and became part of his covenant family.
It's also very interesting that circumcision only applies to maies. Could this have someting to do with the fact that Adam (per my recent argument) was of the serpent's ungodly seed, and so all male descendants of Adam who participate in God's covenant blessings were given a symbolic way to cleanse themselves -- to "separate" themseles from the serpent's ungody seed, which inlcuded Adam? Just a thought....
Now...let's look circumcision under the New Covenant (NC). We find out that true spiritual substance to the religious rite of phsycial circumcision is that like the physical the spiritual is also done to God's chosen people (his elect). This circumcision is performed by the Holy Spirit (Rom 2:29) and by Christ (Col 2:11). Also, this inward, invisible, spiritual work is not the result of man's free moral agency!
I also stronly suspect that this spiritual circumcision is part and parcel of the promised "new heart" NC promise in Jeremiah 31. This circumcision, according to Col 2:11 is a "cutting off of the flesh" (sinful nature) -- the cutting off the heart's callousness (Rom 2:29). It's a removal of the unregenerate person's natural hardness of heart with which they were born. The removal of this hardness renders the heart "new" in that for the first time ever that person's heart has been supernaturally acted upon to make it soft, pliable and sensitive to God and his calling. And I have no dobut at all that this happens during the regeneration process. And we must not forget that this divine action upon the human hearts affects ALL man's faculties -- mind, will, emotions and conscience!
So, now with all this background of both types of circumcision behind us, we still have to answer the question of why would God tell Israel that they had to circumcise their own heart -- under the pains of his wrath, yet, if they disbodyed (Jer 4:4)? What was the purpose of such a command? Why would God command them to do something impossible? And we reasonably have to think that the Jews knew this was an impossible command for them; for they knew what physical circumcision was all about. Their reaction was likely very similar to Nicodemus' incredulity when Jesus told him that he had to be born again! Such hard commands were designed to show the Israleties of the impossibility of their escape from their spiritual plight. They, like Jeremiah, were trapped in the very deep, dark muck and mire of a well from which a solo escape was impossible! They were not going to extricate themselves by the power of their own will! They needed to cry out to YHWH to save them from their impossible situation! Even Paul tells us that one of the purposes behind the Law was to lead the Israelites to their Messiah (Gal 3:24)!
I had much more to say about this issue, but as it is, it has taken me literally hours to bang this out because of numerous interrruptions. But in closing, I will remind everyone of what should be obvious by now: Circumcision under the Old and New Covenants (most especially the latter) is not something that man has control over. It's not within man's power to circumcise himself. That's way above his pay grade. Therefore, it cannot truly be said that God "forces" his will upon any of his chosen people when he does something for or to them that they themselves cannot do!