He only is my rock and my salvation,
My (D)stronghold; I shall not be greatly shaken.
Here again we can see that salvation is found in God alone. I believe that this is one of the points that the psalmist is trying to emphasize in this particular psalm. We cannot seek our salvation in anything else. We also cannot seek our salvation in any combination of God with something else. He is a jealous God, who will not share worship.
But He is such a great salvation. He will not share, because He made us and knows that divided worship won't work. He is a rock and a fortress, when it is He alone. We will not be toppled down as long as we are in Him.