Albert Barnes Commentary Deuteronomy 13:2

"and the sign or the wonder come to pass, whereof he spake unto thee, saying, Let us go after other gods, which thou hast not known, and let us serve them;" — Deuteronomy 13:2 (ASV)

The Lord had said, You shall have no other gods but Me. A prophet is here supposed who invites the people to go after other gods. To such a one no credit is under any circumstances to be given, even if he shows signs and wonders to authenticate his doctrine. The standing rule of faith and practice had been laid down once for all—that the people were to hold fast. The prophet who propounded another rule could only be an impostor.

A different case is considered in Deuteronomy 18:18 and following.