מגדף
Parshat שלח makes mention of the מגדף, who is a blasphemer. He brazenly belittles and challenges the authenticity of the Torah.
Some say that the אפיקורוס, a form of heretic, qualifies to be placed in the category of מגדף. Such an individual does not need a title. He is not ashamed to make such statements that the Torah was not transmitted from Hashem to Moshe. They deny the concept of משיח or תחיית המתים, the resurrection of the dead.
The Torah is very emphatic as to the ultimate fate of the מגדף. The punishment is stated as הכרת תכרת, that his soul shall surely be cut off from the Jewish people. He may die young in addition to losing his share in the next world.
There are other times when כרת is the punishment for various sins. Eating חמץ on Pesach, eating on Yom Kippur, and certain instances of trespassing הקדש, sanctified matters, are all כרת matters.
However, in these cases, the Torah will either say, ונכרתה האיש, or ונכרתה הנפש, that soul or that person, shall be cut off.
It is only the מגדף that has the language of הכרת תכרת. This is a clear indication that this is viewed as a much more serious transgression.