NOTES for Deu 18:1-22
The Book of Deuteronomy gives a great deal of attention to prophets, both Yahwist and pagan. There is nothing surprising about that. The pagan world was full of prophets. Today it sometimes appears to us that prophecy is an exclusively Yahwist phenomenon, that a prophet always speaks in the name of God, the one true God spoken of in the Bible. In fact, prophesying was very widespread in antiquity, and there were prophets who prophesied in the name of many different pagan gods and spirits.
Prophesying is a particular spiritual and psychological phenomenon that, at its base, comes down to an expansion of the boundaries of perception, allowing a person to see and hear what he cannot see and hear in an ordinary state. Such an expansion of boundaries is not necessarily caused by spiritual reasons and processes. It can be a purely psychological phenomenon, and then outwardly the person looks just like a true prophet, but his "prophecies" can be called prophecies only in quotation marks.
Such "prophetic" ecstasy was fairly widespread in antiquity, which is why the sacred writer proposes caution even toward those prophets who prophesy in the name of Yahweh. With pagan prophets everything is clear: to listen to them and follow them means apostasy, with all the consequences that follow. But caution was also needed with Yahwist prophets, or more precisely, with those who claimed that role. Of course, genuine prophets were to appear, and listening to such prophets was not only allowed but directly necessary, as the sacred writer says (the capital letter, with all its semantic connotations here, is purely a matter of translation; there is no basis for it in the original text).
The criterion offered was simple and obvious: if what the prophet says comes true, then the prophet is genuine; if not, then he is not. As we can see, the prophecy had to be completely clear, concrete, and unambiguous, since only in that case could the stated criterion be applied. This is understandable: a prophet was not simply preaching or reflecting on a possible future. He was proclaiming God's will and God's plan. There could be no ambiguity or double meaning here, because God is always completely concrete in His plans and purposes. The witness of a true prophet, conveying God's will and plans to people, is the same.