To Him You Shall Cleave

Siftei Chachamim: "Cleave to His ways" could have been understood as the result of following the instructions which precede it: "If you keep His commandments, listen to His voice, and serve Him, then you shall cleave to Him", i.e. you shall benefit from His Providence. But this passage does not deal with the reward for commandments. Rather it is contrasting what Israel ought to do rather than follow false prophets. This part of the verse thus says that Israel should emulate G‑d rather than adopt any foreign behavior suggested by a false prophet.

Ramban: "keep His commandments"
This negates any consideration of obedience to a false prophet, that we are not to fear his words because of any sign or wonder, and we should not transgress the service of G‑d because of the prophet's words, for it is G‑d alone that we are to fear, and no other god.

Ohr HaChayim: Why did Moshe interrupt the paragraph about the false prophet with this verse? I believe that Moshe wanted to spell out the various elements which comprise serving G‑d. Once we know this, we can better understand the penalty for a false prophet or a dreamer applies even if he asks you to violate only a single one of these elements which are part of serving G‑d by applying them to worship of an alien god.

Tzeror HaMor: Moses knows he is speaking about phenomena that are by definition incapable of being proven by absolute certainty, seeing that he himself is a prophet, and if the words of a prophet had been subject to his performing miracles, as he had done for the people in Egypt and for Pharaoh, why would his words be more believable than future prophets?
They are only to follow the revealed word of G‑d...
That is why he adds, “after G‑d your G‑d”: he tells the people that he does not want them to follow what he says because he is a prophet, neither are they to follow subsequent prophets. They are only to follow the revealed word of G‑d, comparing it with what G‑d had revealed at Mount Sinai...

If ever a prophet arose who would command our people to adopt some idolatrous practice in the name of G‑d, we would know that it was a test of our loyalty. This impostor would deserve the death penalty and he will be executed.

Zohar Vayikra 21:
Rabbi Hiya opened by saying, "To the chief Musician on the morning doe, A psalm of David". (Ps. 22:1) Who is the morning doe? It is the Congregation of Israel called, "a loving ibex and a pleasant roe". (Prov. 5:19) Is a doe in the morning, not during the whole day? The explanation is that the doe is from the place called morning, as in the verse: "His going forth is sure as the morning". (Hosea 6:3) King David said this of the Congregation of Israel, as understood from the words "on the morning doe."

Come and see: When night falls, the closed openings of the upper and lower beings are laying down, and all the far ones rise to walk and roam the world, to go about the bodies of men and around their homes and beds. They see the image of the Holy King and they are afraid they had strengthened themselves in their beds with the words of the Holy Name. People's souls ascend, each as it deserves, as has been already explained. Fortunate is the portion of the righteous whose souls ascend while they sleep and are not needlessly detained in an undesirable area.

At midnight, a herald sounds a proclamation and the gates are opened. A wind from the north side stirs strums on David's harp, which plays by itself and praises the King. The Holy One, blessed be He, delights Himself with the righteous in the Garden of Eden.
Fortunate is the portion of he who wakes up from sleep at that time and labors in the Torah.
Fortunate is the portion of he who wakes up from sleep at that time and labors in the Torah. He is called the friend of the Holy One, blessed be He, and the Congregation of Israel. Moreover, they are called His brethren and companions, as written, "For my brethren and companions' sakes, I will now say, Peace be within you". (Ps. 122:8) They are called companions to the supernal hosts and angels, as written, "The companions hearken for your voice". (Songs 8:13)

When day comes, a herald sounds a proclamation, the gates of the south side are opened and the stars and constellations awaken. The gates of mercy open and the King sits to receive the praises. The Congregation of Israel takes then the words and rises. All the friends cleave to the wings, and their words come to rest in the lap of the King. The King then commands to record all these words.

In the book are recorded all the members of His household and a thread of grace is drawn on them. By this thread of grace, man is crowned with the King's crown, and is feared by the upper and lower beings. He passes through all the King's gates and none dares stop him. Even when the prosecutors judge the world, they do not sentence him, because he is marked with the King's imprint, and it is known that he is of the King's palace. He is therefore not judged. Fortunate is the portion of the righteous who study the Torah, especially when the Holy One, blessed be He, is desirous of words of the Torah.

Come and see: The secret meaning of this is that the Congregation of Israel does not stand before the King unless it is with the Torah. As long as all of Israel in the Land of Israel are occupied with the Torah, the Congregation of Israel dwells among them. When they lay idle of the words of the Torah, the Congregation of Israel cannot be with them even for an instant. Hence, when the Congregation of Israel awakens towards the King with the Torah its (her) power strengthens, and the Holy King is happy to receive her.

When the Congregation of Israel comes before the King without the Torah, she grows weak. Woe to those who weaken the higher power. Therefore, happy are those who do study the Torah, especially when it behooves them to participate with the Congregation of Israel. The Holy One, blessed be He, then declares about them, "and said to me, you are My servant, Israel, in whom I will be glorified". (Isaiah 49:3)