There are several things we can say to summarize Calvin’s discussion about the nature of prayer (III.xx.2):
(1) Prayer brings us the blessings that God has promised.
“It is by the benefit of prayer that we reach those riches laid up for us by the Heavenly Father.”
(2) Prayer yields communion with God.
This is true even when prayer is for our necessities and the blessings God has promised.
(3) This communion with God is real.
Calvin says that when we pray “we appeal to [God] in person.”
(4) Prayer and its answer strengthens our faith.
We are to pray and receive God’s blessings in answer to prayer so that we will know and “experience” that what we believe “is not in vain.”
(5) We are to pray about everything, even those things we can do (and will do) ourselves.
“We see that nothing to us is promised to be expected from the Lord, which we are not also bidden to ask of him in prayers.”
(6) Prayer gives us a taste of heaven by, to a lesser degree, making our faith sight.
“So true is it that we dig up by prayer the treasures that were pointed out by the Lord’s gospel, and which our faith has gazed upon.”