I have already written about this so I will not repeat that.
In Psalm 21:7 there is a different perspective. The Psalmist says that the king who trusts in the Lord will not be shaken. The thing that struck me this time through is that it is through the lovingkindess of the Lord that the king, we are not shaken.
When you put these together I begin to see that only through trust in the Lord and through His lovingkindness am I able to approach the requirements in Psalm 15 and thus it is still through His lovingkindness that I am unshaken by this world.
We are totally, completely, and in all other ways dependent on Him even to follow His commands.