I love you, O LORD, my strength. The LORD is my rock and my fortress and my deliverer, my God, my rock, in whom I take refuge, my shield, and the horn of my salvation, my stronghold.

Psalm 18:1–2

I love you, O LORD, my strength. It is to be observed, that love to God is here laid down as constituting the principal part of true godliness; for there is no better way of serving God than to love him. No doubt, the service which we owe him is better expressed by the word reverence, that thus his majesty may prominently stand forth to our view in its infinite greatness. But as he requires nothing so expressly as to possess all the affections of our heart, and to have them going towards him, so there is no sacrifice which he values more than when we are bound fast to him by the claim of a free and spontaneous love; and, on the other hand, there is nothing in which his glory shines forth more conspicuously than in his free and sovereign goodness. Moses, therefore (Deut. 10:12), when he meant to give a summary of the law, says, “And now, Israel, what does the Lord your God require of you but to love him?” In speaking thus, David, at the same time, intended to show that his thoughts and affections were not so intently fixed upon the benefits of God as to be ungrateful to him who was the author of them, a sin which has been too common in all ages. Even at this day we see how the greater part of mankind enjoy wholly at their ease the gifts of God without paying any regard to him, or, if they think of him at all, it is only to despise him. David, to prevent himself from falling into this ingratitude, in these words makes as it were a solemn vow, Lord, as you are my strength, I will continue united and devoted to you by unfeigned love.

The LORD is my rock, my fortress and my deliverer; my God is my rock, in whom I take refuge. He is my shield and the horn of my salvation, my stronghold. When David thus heaps together many titles by which to honour God, it is no useless or unnecessary accumulation of words. We know how difficult it is for men to keep their minds and hearts stayed in God. They either imagine that it is not enough to have God for them, and, consequently, are always seeking after support and succour elsewhere, or, at the first temptation which assails them, fall from the confidence which they placed in him. David, therefore, by attributing to God various methods of saving his people, protests that, provided he has God for his protector and defender, he is effectually fortified against all peril and assault.

— Adapted from Heart Aflame: Daily Readings from John Calvin on the Psalms