Is it not wheat harvest to day? I will call unto the LORD, and he shall send thunder and rain; that ye may perceive and see that your wickedness is {k} great, which ye have done in the sight of the LORD, in asking you a king.

(k) In that you have forsaken him, who has all power in his hand, for a mortal man.

Wesley's Notes

12:17 Wheat - harvest - At which time it was a rare thing in those parts to have thunder or rain; the weather being more constant in its seasons there, than it is with us. Rain - That you may understand that God is displeased with you; and also how foolishly and wickedly you have done in rejecting the government of that God, at whose command are all things both in heaven and in earth.

Jamieson-Fausset-Brown Bible Commentary

1Sa 12:17-25. He Terrifies Them with Thunder in Harvest-time.

17-25. Is it not wheat harvest to-day?-That season in Palestine occurs at the end of June or beginning of July, when it seldom or never rains, and the sky is serene and cloudless. There could not, therefore, have been a stronger or more appropriate proof of a divine mission than the phenomenon of rain and thunder happening, without any prognostics of its approach, upon the prediction of a person professing himself to be a prophet of the Lord, and giving it as an attestation of his words being true. The people regarded it as a miraculous display of divine power, and, panic-struck, implored the prophet to pray for them. Promising to do so, he dispelled their fears. The conduct of Samuel, in this whole affair of the king's appointment, shows him to have been a great and good man who sank all private and personal considerations in disinterested zeal for his country's good and whose last words in public were to warn the people, and their king, of the danger of apostasy and disobedience to God.

Matthew Henry's Concise Commentary

12:16-25 At Samuel's word, God sent thunder and rain, at a season of the year when, in that country, the like was not seen. This was to convince them they had done wickedly in asking a king; not only by its coming at an unusual time, in wheat harvest, and on a clear day, but by the prophet's giving notice of it before. He showed their folly in desiring a king to save them, rather than God, or Samuel; promising themselves more from an arm of flesh, than from the arm of God, or from the power of prayer. Could their prince command such forces as the prophet could do by his prayers? It startled them very much. Some will not be brought to see their sins by any gentler methods than storms and thunders. They entreat Samuel to pray for them. Now they see their need of him whom shortly before they slighted. Thus many who will not have Christ to reign over them, would yet be glad to have him intercede for them, to turn away the wrath of God. Samuel aims to confirm the people in their religion. Whatever we make a god of, we shall find it deceive us. Creatures in their own places are good; but when put in God's place, they are vain things. We sin if we restrain prayer, and in particular if we cease praying for the church. They only asked him to pray for them; but he promises to do more, to teach them. He urges that they were bound in gratitude to serve God, considering what great things he had done for them; and that they were bound in interest to serve him, considering what he would do against them, if they should still do wickedly. Thus, as a faithful watchman, he gave them warning, and so delivered his own soul. If we consider what great things the Lord hath done for us, especially in the great work of redemption, we can neither want motive, encouragement, nor assistance in serving him.