Adams on Psalm 116:16

16 I am thy servant The saints have ever had a holy pride in being God's servants; there cannot be a greater honour than to serve such a Master as commands heaven, earth, and hell. Do not think thou dost honour God in serving him; but this is how God honours thee, in vouchsafing then to be his servant. David could not study to give himself a greater style than - "O Lord, or, truly I am thy servant, and the son of thy handmaid," and this he spake, not in the phrase of a human compliment, but in the humble confession of a believer. Yea, so doth the apostle commend this excellency, that he sets the title of servant before that of an apostle; first servant, then apostle. Great was his office in being an apostle, greater his blessing in being a servant of Jesus Christ; the one is an outward calling, the other an inward grace. There was an apostle condemned, never any servant of God.

As quoted in Spurgeon's Treasury of David