39. The False Prophet
Yesodei HaTorah 8:3
If a prophet arises and tries to countermand the prophecy of Moses by giving signs and performing wonders, we may not listen to him. We know for sure that he is just performing tricks because the prophecy of Moses does not rely on wonders. Therefore, there’s no point in comparing wonders in order to see whose are more impressive; it’s moot. Everyone saw and heard God appoint Moses. This is like the case of witnesses who give testimony to a man who also observed the situation himself. He has no need listen to them and he will know for sure if they are lying.
Therefore, the Torah tells us, “Even if the sign or wonder happens, do not listen to the words of that prophet" (Deuteronomy 13:3-4). The false prophet brings signs and wonders to deny something you know firsthand. We believe in a prophet’s wonders only because Moses communicated to us a mitzvah that we do so. If so, how could we let a wonder cause us to accept a contradiction of Moses?
Yesodei HaTorah 9:1
It is explicit in the Torah that it is God's word, remaining as it is forever, with no changes, additions or deletions; Deuteronomy 13:1 says, “All these things that I command to you, be careful to perform. Do not add to it or detract from it.” Deuteronomy 29:28 says, “That which is revealed is for us and our children forever, to carry out all the words of this Torah.” From here we see that the Torah's rules apply forever. The Torah also says, “It is an eternal statute for all your generations” (Exodus 27:21), and Deuteronomy 30:12 tells us that “it (i.e., the Torah) is not in the heavens.” We learn from this that a prophet cannot add new mitzvos.
Therefore, if a person arises, whether Jewish or non-Jewish, if he gives a sign or performs a wonder saying that God sent him to add a mitzvah, cancel a mitzvah, or interpret a mitzvah in a manner different from the tradition we received from Moses, or if he claims that the mitzvos do not apply forever, but rather they expire, then we know that he is a false prophet. He denies the prophecy of Moses and the penalty is death by strangulation because he dared to make false statements in God's Name. God told Moses that the Torah is for us and our children forever, and God is not a person that He might lie.