Sforno on Deuteronomy 13:5/ספורנו על דברים י״ג:ה׳

1אחרי ה' אלוקיכם תלכו, similarly phrased to והלכת בדרכיו in Deut. 28,9. You are not to seek out new paths which the “prophet” or “dreamer” employs to make you depart from the ways of the Lord.

2ואותו תיראו, even though the person who now claims to be a prophet had previously been considered as a pious and thoroughly G’d-fearing individual. Your prior esteem of him and his views must turn into total rejection and you must allow your awe and reverence for G’d and His words to outweigh any argument he may present. ואת מצותיו תשמורו, so that you will only observe His commandments and not any new ones claimed to be His by the new “prophet.” This refers in particular to matters involving idolatry where the most minute deviation automatically challenges the totality of Torah and all of its laws.

3ובקולו תשמעון, you are to carry out all the instructions given to you by properly accredited prophets and to listen to their commands, such as to Elijah at Mount Carmel, and not to the instructions of anyone canceling any of G’d’s commandments.

4ואותו תעבודו, exclusively. You cannot simultaneously serve any other deity as spelled out in Exodus 22,19 זובח לאלוהים יחרם, “if anyone is found offering sacrifices to any other deity he is guilty of death.”

5ובו תדבקון, the objective of all your activities must be to carry out His will unaffected by any prior hostility.