John Calvin Commentary Psalms 119:38

"Confirm unto thy servant thy word, Which [is in order] unto the fear of thee." — Psalms 119:38 (ASV)

Confirm your word to your servant. Here we have briefly explained the sole purpose and legitimate use of prayer, which is, that we may reap the fruits of God’s promises. From this it follows, that those who utter vague and incoherent desires commit sin. For we perceive the prophet does not allow himself to petition or wish for anything but what God has condescended to promise. And certainly, great is the presumption of those who rush into the presence of God without any call from His word, as if they would make Him subservient to their whims and caprice.

The argument by which the Psalmist enforces his plea deserves to be noticed; because I am devoted to your fear. The relative אשר asher, in this place has the meaning of the causal conjunction, because or for. The prophet intimates that he does not satisfy himself with mere temporal enjoyments, as worldly men do; and that he did not make a preposterous abuse of God’s promises, to secure the delights of the flesh, but that he made his fear and reverence his aim. And truly, the best assurance we can have of obtaining our requests is when these requests and God’s service harmonize, and our sole desire is that He may reign in and over us.