Albert Barnes Commentary Mark 4:27

"and should sleep and rise night and day, and the seed should spring up and grow, he knoweth not how." — Mark 4:27 (ASV)

And should sleep, and rise night and day. He should sleep at night and rise by day, for this is how the expression is to be understood. That is, he should live in his usual way, without exerting any influence over the growing grain. By this, we are not to infer that people should use no diligence in the attainment and growth of piety; but the illustration shows this, and this only: that just 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 research of philosophers, not 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, examination, searching the Scriptures, and the ordinances of religion, but we cannot tell in what way the religious principle is strengthened. As God, unseen yet by the use of proper means, makes the grass flourish, so God, unseen but by proper means, nourishes the soul, and the plants of piety spring up, bloom, and bear fruit. See John 3:8.