Our Gemara on Amud Aleph references one of the key verses in the Torah that discusses the function and guidelines for testimony (Devarim 19:15):

A single witness may not validate against an [accused] party any guilt or 

blame for any offense that may be committed; a case can be valid only on the testimony of two witnesses or three.
 

The verse could have started from the last half of the verse, stating, “a case can be valid only on the testimony of two witnesses or three.” The first part, by omission, could have been inferred, namely that a single witness is insufficient to convict. Our Gemara derives from this extra phrase that one witness can obligate the defendant to make an oath. The derivation is: True, for any iniquity or for any sin, a single witness is not valid, as the testimony of one witness is not enough for this purpose, but his testimony is valid enough to render one liable to take an oath.

A similar verse is found later in Devarim (17:6), which is also used for various derashos:
 

A person condemned to death shall be put to death only on the testimony of two or more witnesses; no one shall be put to death on the testimony of a single witness.

Be’er Mayim Chaim (ibid) explains these verses allegorically. First, he notes the redundancy, “A person condemned to death shall be put to death.” In the actual Hebrew, the literal reading sounds even more redundant than the translation. Literally, it is, “The dead one shall be put to death.” This strange phrase implies that the person who will be condemned to death is already dead! We will return to that question later. The other feature of this verse is the two and three witnesses. In spiritual life, there can be objects that also serve as witnesses, as Moshe admonishes the Jewish people (Devarim 30:19), “I call heaven and earth to witness against you this day…”

Be’er Mayim Chayim states that every Jew must utilize the testimony of two witnesses daily to counter the forces of evil. These witnesses are the Bris Milah, Tefilin, and Shabbos. On Shabbos, we don’t wear Tefilin, as the second witness is Shabbos itself. These witnesses put the “dead one” to death. The “dead one” is the force of evil, which, by definition, is death due to its detachment from God. However, if the Jewish people are unable to muster two witnesses, even one witness will stimulate an oath. Allegorically, this means that even if the Jews do not keep Shabbos or Tefilin, if they have the Bris Milah, then Hashem will step in to fill the gap by virtue of His oath to our forefathers.

Be’er Mayim Chaim develops the allegory further, but for our purposes, let’s focus on the main idea. There is something about the Bris Milah that activates God’s protection and overcomes evil, even when the other mitzvos are neglected. Indeed, while unfortunately not absolute, to this day, a large majority of Jews, even secular Jews, still choose to circumcise. Even the deranged sycophantic German reform synods of Braunsweig in the 1840’s, which sought to recast Judaism with modern modern German sensibilities, and tried to eliminate Beis Milah, never really caught on regarding that particular “innovation“. 

It’s hard to argue with the fact that Bris Milah is still felt as central to Jewish identity, even more than Shabbos. Why is it so powerful psychologically, socially, and metaphysically? A simple answer is that it is a permanent feature of the body. Also, it is something that the parents undertake at a time in life when it is hard not to feel spiritual. (How can one witness the miracle of birth and not have some sense of transcendence? So to speak, there are no atheists in delivery rooms.) A final point to consider is that the time of the Bris is a time where families and generations converge, physically but also emotionally, as the name of the child is often that of a deceased relative. Even a Jewish soul that is nearly deaf to religious stirrings becomes somehow aroused by this ritual. The Bris Milah has so far withstood the test of time, against all odds and against rationality. I’m afraid to profess confidence that secular Jews will always continue to embrace this covenant, but I pray that they do, for all our sakes.