Charles H. Spurgeon on Psalms 57:7

My heart is fixed, O God, etc. Fitness for duty lies in the orderly temper of body and mind, making a man willing to undertake, and able to finish his work with comfortable satisfaction. If either the body or mind be distempered, a man is unfit for such an undertaking; both must be in a suitable frame, like a well tuned instrument, else there will be no melody: hence when David prepared himself for praises and worship, he tells us his heart was ready and fixed, and then, his tongue was ready also (Ps 45:1), so was his hand with psaltery and harp; all these were awakened into a suitable posture. That a man is or hath been in a fit order for service may be concluded from

1. His alacrity to undertake a duty.

2. His activity in the prosecution.

3. His satisfaction afterward. Right grounds and principles in these things being still presupposed. Richard Gilpin (1625-1699,1700), in "Daemonologia Sacra."