Deuteronomy 18:20

"But the prophet, that shall speak a word presumptuously in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, that same prophet shall die. And if thou say in thy heart, How shall we know the word which Jehovah hath not spoken? when a prophet speaketh in the name of Jehovah, if the thing follow not, nor come to pass, that is the thing which Jehovah hath not spoken: the prophet hath spoken it presumptuously, thou shalt not be afraid of him."

These last verses in the chapter deal with how to distinguish between the true prophets and the false prophets. Dummelow thought that, "At no time is it easy to distinguish between the true prophet and the false prophet,"[33] but it appears to us that he was mistaken in this. It is easy enough to tell the true from the false, whether in the instance of prophets or teachers, and it is just as imperative now as it was then to be able to tell the one from the other. Of course, in these two verses, only one test was proposed, and as Dummelow said, "This test could only be applied to prophecies of the immediate future."[34]

Unger gave the following as a means of distinguishing between true and false prophets:

"(1) The false prophet leads into some form or variation of idolatry.

(2) He speaks his own words (not God's words), and in the name of other gods.

(3) His promises are fake or only half-true. The prophecies of true prophets are true.[35]"

In addition to these tests, the moral character of the prophet himself, or the teacher, and the moral quality of their teachings give invaluable clues. In the N.T., the same question of how to discern the false teacher is treated, and there it all turns upon his evaluation of Christ. "Every spirit that confesses not Jesus, is not of God" (1 John 4:3). To this we might also add that any denial of the Holy Bible is a certain mark and indication of the false teacher, of which we say, sadly, "many of them have gone out into the world."

Notice here how Moses moved immediately to the subject of the false prophets, not adding any explanation whatever. Why? Prophecy was already known throughout the world of that day, and the people already knew of both kinds, the false and the true, having very recently at the time Moses spoke these words stripped the clothes off of the dead Balaam. Thus, there is no "institution of the law of prophecy" here at all, but a promise of "that Prophet" who would redeem people from sin.