He only is my rock - See the notes at Psalm 18:2.
I shall not be greatly moved - The word greatly here, or much - "I shall not be much moved," implies that he did not anticipate perfect security from danger or calamity; he did not suppose that he would escape all disaster or trouble, but he felt that no great evil would befall him, that his most important interests were safe, and that he would be ultimately secure. He would be restored to his home and his throne, and would be favored with future peace and tranquility. None of us can hope wholly to escape calamity in this life. It is enough if we can be assured that our great interests will be ultimately secured; that we shall be safe at last in the heavenly world. Having that confidence the soul may be, and should be, calm; and we need little apprehend what will occur in this world.

I shall not be greatly moved - Having God for my rock - strong fortified place, for my salvation - continual safety, and my defense - my elevated tower, which places me out of the reach of my enemies; I shall not be greatly moved - I may be shaken, but cannot be cast down.

(b) He only [is] my rock and my salvation; [he is] my defence; I shall not be greatly moved.
(b) It appears by the often repetition of this word, that the prophet endured many temptations by resting on God and by patience he overcame them all.

He only is my Rock and my salvation,.... The Rock on which the church is built, and every believer; and which was David's safety, shelter, and shade, and which made him easy in his present state; and he was the author of his salvation, and the rock and strength of it, Psalm 95:1;
he is my defence; or refuge; see Psalm 9:9;
I shall not be greatly moved; or "with much motion", as Kimchi; or "with great motions", as Jarchi: he could not be moved off of the rock on which he was built; nor out of the city of refuge, whither he had betook himself for safety; and though he might be troubled in spirit, and shaken in mind, and staggered in his faith, and fall from some degree of steadfastness of it; yet not fall so as to be utterly cast down, or finally and totally, and so as to perish eternally. Aben Ezra interprets it, "shall not be moved" into the great deep; into the abyss or bottomless pit; and so some of the ancient Midrashes expound of "hell" (g); but much better is the Targum,
"I shall not be moved in a day of great affliction;''
see Acts 20:23.
(g) Vid. Jarchi & Yalkut Simeoni in loc.

The titles applied to God often occur (Psalm 9:9; Psalm 18:2).
be greatly moved-- (Psalm 10:6). No injury shall be permanent, though devised by enemies.