Sforno on Deuteronomy 13:6/ספורנו על דברים י״ג:ו׳

1'יומת, כי דבר סרה על ה, even though he had claimed to speak in the name of the Lord and not in the name of some other deity, something which had convinced him that his crime was not subject to the death penalty. We encounter such a scenario in Jeremiah 26,16 where the elders of Jerusalem do not want to convict Jeremiah to the death penalty because he had spoken in the name of the Almighty G’d, i.e. had not denied His sovereignty, although he was perceived as a false prophet, no one accepting his predictions of doom and his call to repentance.

2להדיחך מן הדרך, even though he had never asked you to abandon G’d, and had only presented an “updated” version of this same G’d’s commandments. He had, however, asked you to depart from “timeless Torah,” making G’d’s laws something that could be changed from time to time, thus making Moses the father figure of all prophets irrelevant. [my words. Ed.]