Parshat שלח discusses the procedure for atoning for sins done unintentionally. Generally, if it was an infraction of a serious offense that carries with it a punishment of death or Karet, the atonement is a sin offering given with proper intent. This offering allows the person to be totally forgiven.
However, if one sins in a brazen, intentional manner, he has blasphemed G-d. The Torah's words are כי דבר ה׳ בזה, "for the word of G-d he has despised." The Torah goes on to explain that such a person will הכרת תיכרת, shall surely be cut off. This is the most severe Karet. He is both cut off from this world as well as the next world.
The Talmud in סנהדרין says that the Torah is referring to an אפיקורס, or heretic. Such an individual openly expresses his doubts that the Forah came from Sinai. He may also claim that some of the Torah was written by Moshe himself, without Hashem's authorization.
The description of this blasphemer, sounds very much like today's bible critics. They have the audacity to openly question the validity of the Torah. They use weak arguments by attempting to take the etymology of specific words in the Torah, like טוטפות, for example. They try to show that the word evolved hundreds of years after Sinai.
The Torah teaches that Hashem has no tolerance for such people who undermine the truth and authenticity of our holy Torah. Such people shall surely be cut off from the face of the earth. Shabbat Shalom