Charles H. Spurgeon on Psalms 143:2

Thy servant. A servant is one who obeys the will of another...There were these four ways in which one might come to be a servant —by birth, by purchase, by conquest, and by voluntary engagement. Some were servants in one of the ways, and some in another. There were servants who were born in the master's house, servants who were bought with the master's money, servants who were the captives of his sword and bow, and servants who had freely engaged themselves to do his work...In the case of the believer there is something that is peculiar and remarkable. He is God's servant by birth. But he is more —he is God's servant by purchase. And that is not all: he is God's servant by conquest. Yes, and by voluntary engagement too. He is the servant of God, not in some one of the four ways, but in all of them together. —Andrew Gray (1805-1861), in "Gospel Contrasts and Parallels."