Deuteronomy 13 · a question people ask
If a prophet's sign comes to pass, does that settle whether to listen to him?
Not in the case this chapter describes. The sign coming to pass is part of the case (13:2), and still "you shall not listen to the words of that prophet" (13:3), because his message is "Let’s go after other gods" (13:2). The chapter says he "has spoken rebellion against the LORD your God" (13:5). It doesn't say how the sign came to pass; it says what the LORD is doing: "testing you, to know whether you love the LORD your God with all your heart and with all your soul" (13:3). Moses used the same verb of the wilderness: the LORD led Israel "to test you, to know what was in your heart, whether you would keep his commandments or not" (8:2). Brown-Driver-Briggs gives the verb as "test, try, prove, tempt [but not in the modern sense of the word]".
The verses
“You shall remember all the way which the LORD your God has led you these forty years in the wilderness, that he might humble you, to test you, to know what was in your heart, whether you would keep his commandments or not.”