Deuteronomy 18:20 "[The prophet]... that shall speak in the name of other gods"

HaShem has chosen prophets as teachers and guides for the Jewish people.

We must listen to a prophet because he is bringing us the words and message of HaShem.

A true prophet is loyal to the Torah.

This prohibition tells us that a person is not allowed to give prophecy in the name of false gods and idols.

If such a prophecy is given, then we immediately know that it is false, (see Negative Mitzvah 27 and 28).