John Calvin on Matthew 20:21

21. In the kingdom. It was worthy of commendation in the sons of Zebedee, that they expected some kingdom of Christ, of which not even the slightest trace was then visible. They see Christ exposed to contempt under the mean aspect of a servant; nay more, they see him despised and loaded with many reproaches by the world; but they are convinced that he will soon become a magnificent king, for so he had taught them. It is unquestionably a noble specimen of faith; but hence we perceive how easily the pure seed is no sooner implanted in our hearts than it becomes degenerate and corrupted; for they imagined to themselves a kingdom which had no existence, and presently committed the folly of desiring the highest places. Since, therefore, this wicked ambition flowed from a general principle of faith, which in itself was highly commendable, we ought to pray, not only that the Lord would open the eyes of our mind, but that he would give us continual direction, and keep our minds fixed on the proper object. We ought also to pray, not only that he would bestow faith upon us, but that he would keep it pure from all mixture.