John Calvin Commentary Psalms 17:6

"I have called upon thee, for thou wilt answer me, O God: Incline thine ear unto me, [and] hear my speech." — Psalms 17:6 (ASV)

I have called upon you, etc. This verb, being in the past tense, denotes a continued act; and therefore, it includes the present time. The Hebrew word כי, ki, which we translate surely, often signifies because, and if it is understood this way in this passage, the meaning will be that David took encouragement to pray because, depending on the promise of God, he hoped his prayers would not be in vain.

But, perhaps, it may be thought preferable to change the tense of the verb, as some do, to give this meaning: I will pray because I have until now experienced that you have heard my prayers. I have, however, chosen the exposition that appears to me simpler.

David, in my judgment, here encourages and animates himself to call upon God from the confident hope of being heard, as if he had said, Since I call upon you, surely, O God, you will not despise my prayers. Immediately after, he beseeches God to bestow on him the blessings for which he told us he had an assured hope.