God is able to make grass grow without sun and rain. He is able to heal without the hand of a doctor. It is possible for him to keep his people nourished without food or drink. However, in his providence, God has chosen to use means and instruments to accomplish his purposes. Rain and sun help make the grass grow, doctors and medicine play a key part in people’s health, and food keeps us nourished. These things are called “secondary causes.” Like the Westminster Confession V.3 says, “God, in his ordinary providence, maketh use of means” (Is. 55:10-11). John Calvin stated this well:
When I spoke of the Providence of God being viewed with its mediums, my meaning was this: If anyone shall have assisted his fellow-man when sunk under an extremity of distress, the deliverance rendered by the hand of man is not a human, but a Divine deliverance. The sun rises day by day; but it is God that enlightens the earth by [its] rays. The earth brings forth her fruits; but it is God that giveth bread, and it is God that giveth strength by the nourishment of that bread. In a word, as all inferior and secondary causes, viewed in themselves, veil like so many curtains the glorious God from our sight (which they too frequently do),the eye of faith must be cast up far higher, that it may behold the hand of God working by all these His instruments.
But in what manner the Providence of God can work, without any medium or instrument at all, Christ taught us by His own example, when He repelled the assaulting Tempter with this shield: “Man doth not live by bread only: but by every word that proceedeth out of the mouth of God doth man live” (Matt. 4:4). For as the Redeemer knew that the power of God needed no external support whatever, so He knew that He could supply that strength without bread, which He is nevertheless mercifully pleased to supply by means of bread.