God was David’s salvation and the strength of his life, as it was by his help that he felt himself safe, and free from the terrors of death. Certainly we find that all our fears arise from this source, that we are too anxious about our life, while we acknowledge not that God is its preserver. We can have no tranquillity, therefore, until we attain the persuasion that our life is sufficiently guarded, because it is protected by his omnipotent power. Weighing, as it were, in scales the whole power of earth and hell, David accounts it all lighter than a feather, and considers God alone as far outweighing the whole.

Let us learn, therefore, to put such a value on God’s power to protect us as to put to flight all our fears. Not that the minds of the faithful can, by reason of the infirmity of the flesh, be at all times entirely devoid of fear; but immediately recovering courage, let us, from the high tower of our confidence, look down upon all our dangers with contempt. Those who have never tasted the grace of God tremble because they refuse to rely on him, and imagine that he is often incensed against them, or at least far removed from them. 

But with the promises of God before our eyes, and the grace which they offer, our unbelief does him grievous wrong, if we do not with unshrinking courage boldly set him against all our enemies. When God, therefore, kindly allures us to himself, and assures us that he will take care of our safety, since we have embraced his promises, or because we believe him to be faithful, it is fitting that we highly extol his power, that it may ravish our hearts with admiration of himself. 

—John Calvin, Commentary on Psalms - Volume 3