“One should be as careful with a light mitzvah as with heavy mitzvah” (Avot 2:1). Contrary to what is often taught, not all mitzvot are created equal. Some are more important, some less so. The mitzvah to accept upon oneself to observe the commandments (done through the recital of the shema) is clearly of greater importance than, say, ensuring we put salt on all our sacrifices.
Yet one who wants to maximize the impact of Torah makes sure to carefully observe all mitzvot, bringing the same energy, approach and commitment to a relatively unimportant mitzvah—say, davening ma’ariv—as to the mitzvah of shofar. Such does not mean that we equate the two—that would be ludicrous—but that we take each most seriously. As the Mishna notes, ultimately “we do not know the reward for mitzvot”. Mitzvot that may seem unimportant, and mitzvot that may, in fact, actually be of lesser importance can, due to all kinds of factors, turn out to be of much greater significance than appearances might warrant. A nice, friendly greeting may, unbeknownst to you, be of greater importance than putting on tefillin that day.
The Meshech Chochmah (Shemot 12:22) notes that the importance of a mitzvah can vary due to historical circumstances. While the mitzvah to live in Israel is the same mitzvah today as it was in the 12th century, there is no comparing the importance and impact of the two. At times it is peripheral mitzvoth that take on greater significance, more so than actual mitzvot of the Torah. Even a Jewish practice that fulfils no mitzva, such as a Magen David worn by a non-observant Jew, may have more impact than their attendance at shul on Yom Kippur. Perhaps it is for these very reasons the Torah does not mention specific rewards for performance of positive mitzvot.
Such is not the case regarding the mitzvot lo ta’ase, those activities that a G-d-fearing Jew must avoid. Here, the Torah very clearly spells out the consequences, depending on the severity of the offence. The most serious—murder, idolatry, and adultery, to name three—carry a death penalty; those a bit less severe, for example, eating untithed foods, death at the hands of heaven. Those that involve an action—fully 207 of the 365 negative commands—are punished (in theory) by lashes; and those that involve no action, for example, owning chametz during Pesach, carry no earthly penalty at all.
This brings us to masechet Keritot, which gets its name from the opening Mishna that lists the 36 mitzvot, some six percent of all mitzvot, whose violation renders one liable for the punishment of karet. The Torah never defines what exactly it means by the term karet; it just tells us that one who violates such a prohibition will be v’nechretu, cut off, from the Jewish people. While our commentaries debate whether this means that one will die before one’s time or even worse, lose one’s share in the World to Come, the Torah itself teaches that it is being separated from the Jewish people that is the ultimate punishment.
Only two of these 36 keritot are for the non-observance of positive mitzvot: brit milah, a mitzvah that demonstrates our personal acceptance of the Divine covenant, and failing to offer the korban pesach, a mitzvah that demonstrates our commitment to Jewish peoplehood. Not surprisingly, one who neglects the mitzvah of milah may not eat from the korban pesach.
The other 34 keritot are reserved for what, in the Torah’s view, are the most grievous of sins. Fifteen of them are a result of various sexual improprieties, five relate to idolatry and seven relate to improper eating; comprised of eating food on Yom Kippur (with a second karet for those who do melacha on Yom Kippur); chametz on Pesach; blood; forbidden fats of an animal; leftover sacrifices; meat from a disqualified sacrifice; or sacrificial food in a state of impurity. In addition, one receives karet for entering the Temple in a state of impurity; slaughtering or offering sacrifices outside of the Temple precincts; making or sprinkling the oil used to anoint kings and high priests; and making imitation incense. Blurring the line between the holy and less holy is, in the Torah’s view, a most serious offence.
Karet is decreed only when one purposely violates one of the above prohibitions. If done accidentally, one brings a korban chatat, a sin offering. If one is uncertain as to whether one has violated an issur karet, one brings an asham talui, a sin offering in abeyance. For example, if there are two pieces of meat, one of which is permitted fats and one prohibited, and one is unsure as to which one was eaten. If and when one discovers that they have, in fact, eaten the prohibited fats, one would then bring a korban chatat.
If one enters the Temple or eats sacrificial meat while tameh, a korban oleh v’yored is brought instead. This unique type of sin offering varies according to the wealth of the sinner, with the wealthy bringing an animal; the middle class, birds; and the poor, flour. A variation of this korban—where those who can afford it bring an animal, and those who cannot bring birds, with no option for flour—is brought by a woman after childbirth and by a metzora at the end of his period of tzara’at.
It is the details of these korbanot that serve as the main subject matter of our masechet.
 As examples of a “light mitzvah”, the Rambam in his commentary to the Mishna lists the speaking of Hebrew, which (technically speaking) is not even a mitzvah, and simchat haregel, the biblical mitzvah to be happy on our festivals.
 It is unlikely that we would have declared this to be such a serious sin, more so than eating pork or mixing meat and milk (or a host of other, non-food-related offences). Yet one who does not tithe demonstrates a serious lack of gratitude for one’s blessings—and is sinning both against G-d, the provider of our food, and man, who is entitled to such.
 If we accept this view (Rambam, Hilchot Teshuva 8:1), it turns out there are additional mitzvot that carry the punishment of karet. Curiously, every one of the 36 keritot mentioned in the Mishna is in the realm of mitzvot bein adam lemakom, between man and G-d. Our Sages teach that there are sins against our fellow man so grievous that they too cause one to lose their share in the World to Come, for example one who embarrasses others in public.
 Included in those related to idolatry is the violation of Shabbat. In Talmudic times, such violation was tantamount to denial of G-d. Only those who did not believe that G-d created the world would dare violate the Shabbat. With the advent of modernity such is no longer true, and many a religious—if not observant—Jew violates the Shabbat. It is this social change which allows for a much more tolerant view of Shabbat violators today.
 The olah v’yored is also brought for the non-karet sins of violating an oath or swearing falsely in asserting that one knows nothing when subpoenaed to testify.