For though God has promised to do whatsoever his people may ask, yet he does not allow them an unbridled liberty to ask whatever may come to their minds; but he has at the same time prescribed to them a law according to which they are to pray. And doubtless nothing is better for us than this restriction; for if it was allowed to every one of us to ask what he pleased, and if God were to indulge us in our wishes, it would be to provide very badly for us. For what may be expedient we know not; nay, we boil over with corrupt and hurtful desires. But God supplies a twofold remedy, lest we should pray otherwise than according to what his own will has prescribed; for he teaches us by his word what he would have us to ask, and he has also set over us his Spirit as our guide and ruler, to restrain our feelings, so as not to suffer them to wander beyond due bounds. For what or how to pray, we know not, says Paul, but the Spirit helpeth our infirmity, and excites in us unutterable groans. (Romans 8:26.) We ought also to ask the mouth of the Lord to direct and guide our prayers; for God in his promises has fixed for us, as it has been said, the right way of praying.
John Calvin, Commentaries on the Catholic Epistles