Commentaries:
Once we recognize that the psalmist uses "hills" as an image of a problem to a pilgrim, we easily understand the rest of the psalm as an encouragement to those on a spiritual pilgrimage to the Kingdom of God. It certainly does not give the impression that God—at any time—lets His mind wander, unaware of what is happening in His children's lives. Not only is He ever on guard, He is also ready, willing, and able to intervene with strength. He is not a parent distracted by other concerns so that He neglects His children. We can be greatly encouraged that God is always alert to His responsibilities.
Because God is this way, all things work together for our good, even though there are times when we sin and nothing evil appears to have happened to us. Human nature easily deceives itself into thinking it has gotten away with something. This, however, is like saying we can defy the law of gravity, and nothing will happen! God does not let us get away with anything pertaining to His purpose, but He is never overbearing in thoroughly following through.
John W. Ritenbaugh
The Sovereignty of God: Part Nine
Other Forerunner Commentary entries containing Psalms 121:1:
Luke 11:5-7
Philippians 4:6