And should sleep, and rise night and day - Should sleep in the night and rise by day, for so the expression is to be understood. That is, should live in his usual way, without exerting any influence over the growing grain. By this we are not to infer that men are to use no diligence in the obtaining and in the growth of piety; but the illustration shows only that as we cannot tell how grain grows, so we cannot tell the mode in which piety increases in the heart.
He knoweth not how - This is still true. After all the researches of philosophers, no one has been able to tell the way in which grain grows. They can observe one fact after another; they can see the changes; they can see the necessity of rains and suns, of care and shelter, but beyond this they cannot go. So in religion. We can mark the change; we can see the need of prayer, and self-examination, and searching the Scriptures, and the ordinances of religion, but we cannot tell in what way the religious principle is developed and strengthened. As God unseen, yet by the use of proper means, makes the grass to flourish, so God unseen, but by proper means, nourishes the soul, and the plants of piety spring up, and bloom, and bear fruit. Compare the notes at John 3:8.