Verses 21, 22. - The test by which it was to be discovered which was the true prophet and which the false, was the fulfillment or non-fulfillment of his prediction. The reference here is to the prediction of proximate events - events that were to happen within a limited period, but which were not such as one not divinely instructed could foresee. When such came to pass, the pretensions of the prophet were thereby substantiated, and his authority established (cf. 1 Samuel 3:19; John 2:18, etc.). This was a more certain test than such as was offered by signs and wonders (Deuteronomy 13:2, etc.).

18:15-22 It is here promised concerning Christ, that there should come a Prophet, great above all the prophets; by whom God would make known himself and his will to the children of men, more fully and clearly than he had ever done before. He is the Light of the world, Joh 8:12. He is the World by whom God speaks to us, Joh 1:1; Heb 1:2. In his birth he should be one of their nation. In his resurrection he should be raised up at Jerusalem, and from thence his doctrine should go forth to all the world. Thus God, having raised up his Son Christ Jesus, sent him to bless us. He should be like unto Moses, only above him. This prophet is come, even JESUS; and is He that should come, and we are to look for no other. The view of God which he gives, will not terrify or overwhelm, but encourages us. He speaks with fatherly affection and Divine authority united. Whoever refuses to listen to Jesus Christ, shall find it is at his peril; the same that is the Prophet is to be his Judge, Joh 12:48. Woe then to those who refuse to hearken to His voice, to accept His salvation, or yield obedience to His sway! But happy they who trust in Him, and obey Him. He will lead them in the paths of safety and peace, until He brings them to the land of perfect light, purity, and happiness. Here is a caution against false prophets. It highly concerns us to have a right touchstone wherewith to try the word we hear, that we may know what that word is which the Lord has not spoken. Whatever is against the plain sense of the written word, or which gives countenance or encouragement to sin, we may be sure is not that which the Lord has spoken.And if thou say in thine heart, &c. Such a thought arises in the mind, and it appears to be a difficulty, and a query is made upon it:

how shall we know the word which the Lord hath not spoken? What marks, signs, and criterions are those by which it may be known that it is not a word that comes from the Lord?