John Calvin Commentary Mark 4:26

"And he said, So is the kingdom of God, as if a man should cast seed upon the earth;" — Mark 4:26 (ASV)

So is the kingdom of God. Though this comparison has the same object as the two immediately preceding, Christ appears to direct his discourse purposely to the ministers of the word, so that they may not grow indifferent about the discharge of their duty, because the fruit of their labor does not immediately appear.

He holds out for their imitation the example of farmers, who throw seed into the ground with the expectation of reaping, and do not torment themselves with uneasiness and anxiety, but go to bed and rise again; or, in other words, pursue their ordinary and daily toil, until the corn reaches maturity in due season.

In like manner, though the seed of the word may be concealed and choked for a time, Christ enjoins pious teachers to be of good courage, and not to allow their alacrity to be diminished by distrust.