In his Institutes of the Christian Religion, John Calvin gives us 6 reasons the providence of God does not render prayer unnecessary.
“But, someone will say, does God not know, even without being reminded, both in what respect we are troubled and what is expedient for us, so that it may seem in a sense superfluous that he should be stirred up by our prayers—as if he were drowsily blinking or even sleeping until he is aroused by our voice? But they who thus reason do not observe to what end the Lord instructed his people to pray, for he ordained it not so much for his own sake as for ours. Now he wills—as is right—that his due be rendered to him, in the recognition that everything men desire and account conducive to their own profit comes from him, and in the attestation of this by prayers. But the profit of this sacrifice also, by which he is worshiped, returns to us. Accordingly, the holy fathers, the more confidently they extolled God’s benefits among themselves and others, were the more keenly aroused to pray. It will be enough for us to note the single example of Elijah, who, sure of God’s purpose, after he has deliberately promised rain to King Ahab, still anxiously prays with his head between his knees, and sends his servant seven times to look [1 Kings 18:42], not because he would discredit his prophecy, but because he knew it was his duty, lest his faith be sleepy or sluggish, to lay his desires before God.
Therefore, even though, while we grow dull and stupid toward our miseries, he watches and keeps guard on our behalf, and sometimes even helps us unasked, still it is very important for us to call upon him:
- First, that our hearts may be fired with a zealous and burning desire ever to seek, love, and serve him, while we become accustomed in every need to flee to him as to a sacred anchor.
- Secondly, that there may enter our hearts no desire and no wish at all of which we should be ashamed to make him a witness, while we learn to set all our wishes before his eyes, and even to pour out our whole hearts.
- Thirdly, that we be prepared to receive his benefits with true gratitude of heart and thanksgiving, benefits that our prayer reminds us come from his hand [cf. Ps. 145:15–16].
- Fourthly, moreover, that, having obtained what we were seeking, and being convinced that he has answered our prayers, we should be led to meditate upon his kindness more ardently.
- And fifthly, that at the same time we embrace with greater delight those things which we acknowledge to have been obtained by prayers.
- Finally, that use and experience may, according to the measure of our feebleness, confirm his providence, while we understand not only that he promises never to fail us, and of his own will opens the way to call upon him at the very point of necessity, but also that he ever extends his hand to help his own, not wet-nursing them with words4 but defending them with present help.
On account of these things, our most merciful Father, although he never either sleeps or idles, still very often gives the impression of one sleeping or idling in order that he may thus train us, otherwise idle and lazy, to seek, ask, and entreat him to our great good.
Therefore they act with excessive foolishness who, to call men’s minds away from prayer, babble that God’s providence, standing guard over all things, is vainly importuned with our entreaties, inasmuch as the Lord has not, on the contrary, vainly attested that “he is near … to all who call upon his name in truth” [Ps. 145:18, cf. Comm. and Vg.]. Quite like this is what others prate: that it is superfluous for them to petition for things that the Lord is gladly ready to bestow, while those very things which flow to us from his voluntary liberality he would have us recognize as granted to our prayers. That memorable saying of the psalm attests this, and to it many similar passages correspond: “For the eyes of the Lord are upon the righteous, and his ears toward their prayers” [1 Peter 3:12; Ps. 34:15; cf. 33:16, Vg.]. This sentence so commends the providence of God—intent of his own accord upon caring for the salvation of the godly—as yet not to omit the exercise of faith, by which men’s minds are cleansed of indolence. The eyes of God are therefore watchful to assist the blind in their necessity, but he is willing in turn to hear our groanings that he may the better prove his love toward us. And so both are true: “that the keeper of Israel neither slumbers nor sleeps” [Ps. 121:4, cf. Comm.], and yet that he is inactive, as if forgetting us, when he sees us idle and mute.”
 Calvin, J. (2011). Institutes of the Christian Religion & 2. (J. T. McNeill, Ed., F. L. Battles, Trans.) (Vol. 1, pp. 851–853). Louisville, KY: Westminster John Knox Press.