According to Orthodox tradition, the laws of taharat hamishpachah (family purity) are arguably the most important commandments. Yet the vast majority of Jews know very little about the rules of mikvah, the Jewish ritual bath, which lies at the center of this obligation.

Many misconceptions and mystery surround this commandment. But the mikvah’s importance in halachah has been linked to the very survival of the Jewish people because it dictates when a husband and wife are permitted to have sexual relations. Following her menstruation, a woman counts seven “spotless” days. During this entire time, from the onset of menstruation until mikvah immersion, couples abstain from marital relations. The name for a woman in this state is niddah, which means to be separated.

Rivkah Slonim of Binghamton, N.Y., is one of the world’s renowned experts on the mikvah ritual. The educational director of Chabad House, she is editor of and contributing writer to the mikvah anthology “Total Immersion.” In the anthology, Slonim writes that it is precisely because it is a chok (a commandment that does not have a comprehensible reason, as opposed to a mishpat, which is open to human understanding, such as “Thou shalt not steal”), that the mikvah is such a potent mitzvah.

“The laws of family purity are a divine ordinance,” she writes. “There is no better, more legitimate, more logical or essential reason for their observance … Ironically this ‘unfathomable’ mitzvah reveals its blessings to us more than almost any other, in daily, palpable ways.”

The mikvah was at the heart of Jewish life when the Temple was in existence and “the only way you could enter the Temple Mount was to be in a state of purity.”

Purity, or taharah, and impurity, tumah, were of the utmost importance during the days of the Temple. Slonim made it clear, however, that the terms have nothing to do with physical cleanliness.

“These are statements about one’s spiritual status,” said Slonim. “Unfortunately, too many people link physical hygiene and mikvah in their minds. There seems to be this inextricable bond between water and hygiene. I think because mikvah is such a mythical concept, many people figure it must be about cleansing. But nothing can be further from the truth.”

In fact, the woman who is about to enter the mikvah is probably “among the cleanest on the face of this earth” because of the vigorous preparation process leading up to immersion. This includes soaping, bathing for a half hour, combing hair, cleaning the fingernails, removing Band-Aids, jewelry and deodorant so that there’s nothing between the woman and the water in the mikvah.

Mikvahs have been called archaic, misogynistic and patriarchal. Slonim, a self-described Chassidic feminist, says nothing could be further from the truth. In fact, she says, many view it as a quintessential feminist rite.

“I think that with more education people are understanding that Judaism has laws that are in part based on a woman’s cycle and that it’s not anti-woman. It’s simply part of a larger system that values, appreciates and celebrates the various demarcations and the various cycles in our lives and in our bodies,” Slonim said.

So what is a mikvah?

Technically, the world’s natural bodies of water — oceans, rivers, spring-fed lakes — are all valid mikvahs. However, for obvious reasons like accessibility, privacy and safety, so-called “domesticated mikvahs” have been in use for thousands of years. (One of the oldest known mikvahs is at Masada.) There are stringent laws as to what constitutes a kosher mikvah.

“In order to be a kosher mikvah,” said Slonim, “it needs to be filled with rainwater that is filtered into the mikvah in a very specific way. It’s a very complex thing and must be monitored very closely.

“A mikvah looks like a tiny little pool, but it’s very deceptive.” A bath, Jacuzzi, a shower — none of these can serve as a mikvah.

In her book, Slonim briefly enumerates some of the many other rules regarding a kosher mikvah:

A mikvah must be built into the ground or built as an essential part of the building (hence, why bathtubs or whirlpools cannot function as mikvahs).

The mikvah must contain a minimum of 200 gallons of rainwater that was gathered and siphoned into the mikvah pool in accordance with a highly specific set of regulations.

Mikvah visits, which include the preparation and immersion, take a few hours of precious time each month. It also means that couples who follow taharat hamishpachah cannot be intimate for two weeks every month.

“It is a discipline, it’s not easy,” concurred Slonim. “I always tell students if it ever becomes easy, run, don’t walk to a marriage counselor! It shouldn’t be easy. But neither is keeping kosher or keeping Shabbos. They’re disciplines. And once you take on the discipline it can be very enriching, it can be a wonderful enhancement to life, but I would never call it easy.”

Being off limits to one another for two weeks out of every month can keep the spark alive in a marriage, she said.

“I want to be careful not to imply that a mikvah will heal all and that a mikvah is a cure-all,” said Slonim. “On the other hand, all things being equal, I feel very comfortable saying that the mikvah infuses a marriage with a particular energy and a particular dynamic that I think would be otherwise impossible or elusive to achieve. And that is because the couple has to find other ways of expressing their love or affection and empathy and sympathy and even upset with the other, for two weeks out of every month without touching skins.

“When a couple can’t have each other whenever they want, when there are punctuation marks, it really creates excitement.”

Mikvah, according to Jewish law, is the porthole to the future. “It’s as simple as that,” she said Slonim, “because there cannot be marriage and there cannot be progeny without immersion in the mikvah.”

To further emphasize the importance of the mikvah to the centrality of Jewish life, Slonim pointed out that if a community has money to build either a synagogue or a mikvah, the mikvah takes precedence.