Where did this practice come from? Probably halachah leMosheh miSinai (the law of Moses from Sinai): in other worlds, Oral Torah.
Whenever something’s source or purpose can not be found, it is inevitably justified by appealing to halachah leMosheh miSinai. This includes not only the practice under discussion but also other elements (my favorite being the rule that the shin on the tefillin shel rosh must have four prongs instead of the usual three). There may be many theories as to where it came from, why it exists, but these are all theories without clear answers. This is not some minor issue, though: it has to do with a core belief of Rabbinic Judaism (the dominant form of Judaism: other forms include Karaite Judaism, Samaritan “Judaism,” and maybe Falasha Judaism). This is expressed most succinctly by the Mishna:
The Men of the Great Assembly would eventually play a hand in the collection, transmission, and codification of the Oral Torah, which now exists in the Talmud. Their teachings are considered to be as authoritative as the text of the Written Torah itself, if not more so. While the Written Torah dictates, the Oral Torah fills in the (many) blanks, addressing issues that the Written Torah does not. (Rules about kashrut, mezuzot, tefillin, tefillah, Festivals, taharat hamishpachah, and so on, are all addressed by the Oral Torah.)
Orthodox Jews do not question so much why such-and-such practice or rule came about. What the Torah teaches, they do. Questions may enhance their understanding and joy of the mitzvot, but finding answers is never used to base whether one will or will not obey a certain mitzvah. This is also why Orthodox Jews expect baalei teshuvah (Jews returning to observance and faithfulness) and converts to, above all, accept, embrace, and obey mesorah or tradition. Orthodox Jews cannot sit and ponder the wherefores of all the mitzvot, and they simply are forbidden to base their obedience or faithfulness based on such questions. They must accept, embrace, and obey mesorah even if it does not make sense. These Jews believe firmly that the Above’s wisdom, intelligence, insight, and knowledge far surpasses their own: who are they to question what He has decreed (as they see it)?
Modern Judaism is so based on Rabbinic Judaism that it is really futile to conjecture, within Judaism, why this practice (or others like unto it) exists, how it came about, what relation it has to the Torah and to the Jewish people, etc. This is much like the Above: it was, it is, and it will be (as long as there are shomer Torah Jews). It is an inexplicable phenomenon. It would be interesting to consider this from an academic/anthropological/sociological point of view: but from a Jewish point of view, it would be very, very difficult to do so while honoring the Rabbinic tradition in the way that it deserves to be.
The practice Rufus Xavier referred to is called hatafat dam brit. This involves drawing a drop of blood from the shaft, followed by immersion in a mikvah or special pool of water. This has less to do with one’s Jewishness and more with complying with the Oral Torah (which implicitly implies the acceptance of Rabbinic Judaism and its structures and teachings).
[hijack] Baptism was not invented by Christians: it’s a very ancient practice, and is used by Jews. Immersion is done to make oneself ritually pure. Many frum Jews, male and (of course) female will make often use of the mikvah. Women are the usual users the mikvah, to fulfill requirements of taharat hamispachah, which is taken very seriously by male and female Orthodox Jews. Outside of very observant Jews, the mikvah is used for women and for converts. The frum use of the mikvah reminds one of the very frequent use of the mikvah by the Essene/Qumran Jews: they used it frequently to keep themselves in a state of ritual purity so that they could fight with the angels in the upcoming battle between the Sons of Light and the Sons of Darkness. Being holy beings, angels would not come near ritually impure people. It should be noted that ritual impurity has little to do with physical or moral cleanliness. Well, a bit with physical cleanliness. This very same idea, of ritual purity, exists in almost all religions of the world, except for Christianity (and maybe Buddhism?), and they all share uncanny resemblances and similarities. [/hijack]
WRS - Not a Jew, although often He wishes He were.
If We are wrong anywhere above, please correct Us.