12. And in thy mercy, etc. In this verse he repeats for the fifth or sixth time that he looked for life only of God’s free mercy. Whatever severity may appear on the part of God when he destroys the wicked, David affirms that the vengeance taken upon them would be a proof of fatherly mercy to him. Indeed these two things often meet together — the severity and the goodness of God; for in stretching out his hand to deliver his own people, he directs the thunder of his indignation against their enemies. In short, he comes forth armed for the deliverance of his people, as he says in Isaiah,
“The day of vengeance is in mine heart, and this is the year of my redemption.” (Isaiah 63:4.)
In calling himself The servant of God, he by no means boasts of his services, but rather commends the grace of God, to whom he owed this privilege. This is not an honor to be got by our own struggles or exertions — to be reckoned among God’s servants; it depends upon his free choice, by which he condescends before we are born to take us into the number and rank of his followers, as David elsewhere declares still more explicitly —
“I am thy servant, truly I am thy servant, and the son of thine handmaid.” (Psalms 116:16.)
This is equivalent to making himself God’s client, and committing his life to his protection.