Rabbi Yisrael Salanter was asked why this section follows Parashat Sh’mini, which sets out the laws of prohibited foods.
He said that the Torah wished to show that just as we must be vigilant in what we eat, so must we be vigilant in adhering to the laws of prohibited speech. Just as forbidden food can make a person impure, so too can the words which we utter. What enters the mouth is important: so is what comes out.
This is not only a vital lesson but also a rebuttal of the New Testament statement (Matt. 15:11; Mark 7:15), “Not that which goes into the mouth makes a person impure, but that which comes out of the mouth”. Judaism always insisted that a person’s diet affects their health, not only of the body but of mind and soul too.