For the king trusteth. Here again the pious Israelites glory that their king shall be established, because he relies upon God; and they express at the same time how he relies upon him, namely, by hope or trust. I read the whole verse as one sentence, so that there is but one principal verb, and explain it thus:- The king, as he places by faith his dependence on God and his goodness, will not be subject to the disasters which overthrow the kingdoms of this world. Moreover, as we have said before, that whatever blessings the faithful attribute to their king, belong to the whole body of the Church, there is here made a promise, common to all the people of God, which may serve to keep us tranquil amidst the various storms which agitate the world. The world turns round as it were upon a wheel, by which it comes to pass, that those who were raised to the very top are precipitated to the bottom in a moment; but it is here promised, that the kingdom of Judah, and the kingdom of Christ of which it was a type, will be exempted from such vicissitude. Let us remember, that those only have the firmness and stability here promised, who betake themselves to the bosom of God by an assured faith, and relying upon his mercy, commit themselves to his protection. The cause or the ground of this hope or trust is at the same time expressed, and it is this, that God mercifully cherishes his own people, whom he has once graciously received into his favor.

For the king - David, the author of the psalm.
Trusteth in the Lord - All these blessings have resulted from his confiding in God, and looking to him for his favor and protection.
And through the mercy of the Most High - The favor of Him who is exalted above all; the most exalted Being in the universe. The word "mercy" here is equivalent to "favor." He had already experienced God's favor; he looked for a continuance of it; and through that favor he was confident that he would never be shaken in his purposes, and that he would never be disappointed.
He shall not be moved - He shall be firmly established. That is, his throne would be firm; he himself would live a life of integrity, purity, and prosperity; and the promises which had been so graciously made to him, and which extended so far into the future, would all be acomplished. The truth taught here is, that however firm or prosperous our way seems to be, the continuance of our prosperity, and the completion of our hopes and our designs, depend wholly on the "mercy" or the favor of the Most High. Confiding in that, we may feel assured that whatever changes and reverses we may experience in our temporal matters, our ultimate welfare will be secure. Nothing can shake a hope of heaven that is founded on his gracious promises as made through a Saviour.

The king trusteth in the Lord - It was not by my skill or valor that I have gained this victory, but by faith in the strong protecting, and conquering arm of Jehovah.
He shall not be moved - Perhaps this may be best understood of him who was David's prototype. His throne, kingdom, and government, shall remain for ever.

For the King trusteth in the Lord,.... That is, the King Messiah, as the Targum paraphrases it; he trusted in the Lord for his support and sustenance as man, for assistance and help in his time of trouble, and for deliverance out of it; he trusted in the Lord that he would hear him for himself, and for his people; and that he would glorify him with all glory, honour, majesty, and blessedness, before spoken of; see Psalm 22:8;
and through the mercy of the most High he shall not be moved; God the Father is the most High; Christ is called the Son of the Highest, and the Spirit the power of the Highest, Luke 1:32; there is mercy with him, which is a ground of hope and trust, in his people, and also in the Messiah; see Psalm 89:28; and some versions make the mercy of the most High to be what the King Messiah trusts in, reading the words (b), "for the King trusteth in the Lord, and in the mercy of the most High"; but the accent "athnach", which distinguishes the propositions, will not admit of it; but the sense is, that because of the mercy, grace, goodness, and faithfulness of God in making and keeping his promises, Christ would not be and was not moved from his trust and confidence in the Lord; nor shall he even be removed from his throne of glory on which he sits; nor from the glorious and happy state in which he is: nor will it ever be in the power of his enemies to displace him; for these in time will be destroyed by him, as the following words show.
(b) So Genebrard, Muis,

The psalmist teaches to look forward with faith, and hope, and prayer upon what God would further do. The success with which God blessed David, was a type of the total overthrow of all Christ's enemies. Those who might have had Christ to rule and save them, but rejected him and fought against him, shall find the remembrance of it a worm that dies not. God makes sinners willing by his grace, receives them to his favour, and delivers them from the wrath to come. May he exalt himself, by his all-powerful grace, in our hearts, destroying all the strong-holds of sin and Satan. How great should be our joy and praise to behold our Brother and Friend upon the throne, and for all the blessings we may expect from him! yet he delights in his exalted state, as enabling him to confer happiness and glory on poor sinners, who are taught to love and trust in him.

The mediate cause is the king's faith, the efficient, God's mercy.

(Hebrews.: 21:8-9) With this strophe the second half of the Psalm commences. The address to God is now changed into an address to the king; not, however, expressive of the wishes, but of the confident expectation, of the speakers. Hengstenberg rightly regards Psalm 21:8 as the transition to the second half; for by its objective utterance concerning the king and God, it separates the language hitherto addressed to God, from the address to the king, which follows. We do not render Psalm 21:8: and trusting in the favour of the Most High - he shall not be moved; the mercy is the response of the trust, which (trust) does not suffer him to be moved; on the expression, cf. Proverbs 10:30. This inference is now expanded in respect to the enemies who desire to cause him to totter and fall. So far from any tottering, he, on the contrary, makes a victorious assault upon his foes. If the words had been addressed to Jahve, it ought, in order to keep up the connection between Psalm 21:9 and Psalm 21:8, at least to have been איביו and שׁנאיו (his, i.e., the king's, enemies). What the people now hope on behalf of their king, they here express beforehand in the form of a prophecy. מצא ל (as in Isaiah 10:10) and מצא seq. acc. (as in 1-Samuel 23:17) are distinguished as: to reach towards, or up to anything, and to reach anything, attain it. Supposing ל to represent the accusative, as e.g., in Psalm 69:6, Psalm 21:9 would be a useless repetition.