1 Samuel 12:16

That mighty deed Samuel is said to have accomplished through prayer is something that everyone who genuinely relies on God can accomplish spiritually even now, since he has become worthy of being heard.… For every saint and genuine disciple of Jesus is told by the Lord, "Lift up your eyes, and see how the fields are already white for harvest. He who reaps receives wages and gathers fruit for eternal life." In this time of harvest the Lord does "a great thing" before the eyes of those who hear the prophets. For when the one adorned with the Holy Spirit calls to the Lord, God gives from heaven thunder and rain that waters the soul, so that he who once was in evil may stand in great awe of the Lord and his minister of goodness, manifested as venerable and august by the requests that are heard. And Elijah, who shut up heaven for the wicked for three and a half years, later opened it. This, too, is always accomplished for everyone who through prayer receives the rain of the soul, since the heavens were previously deprived of it because of his sin.

Therefore it is well added: "But now also stand and see this great thing which the Lord will do before your eyes. Is it not the wheat harvest today? I will call upon the Lord, and He will send thunder and rain; and you shall know and see that you have done a great evil for yourselves in the sight of the Lord, in asking for a king over you." 9. Because the petition of the king is reproved so attentively and so many times, this suggests to the elect that those who advance carnal men to the height of the holy Church are held by a most grave obligation of sin. For other sins obtain the merit of a single punishment, but he who appoints a carnal leader incurs as many merits of punishment as that man provides examples of depravity to the faithful subjects. But the heavy weight of crime cannot be set down except by the severity of great compunction. Yet the grace of compunction is not poured into the mind unless the magnitude of the sin is first shown to it. Therefore the prophet of the Lord, in order to provoke the sinning people to the fruit of fitting penance, suggests to them the great sin of transgression through a wondrous disturbance of the air. Now in that region, rains and thunders do not occur during the wheat harvest.

But Samuel made it to appear that this was designed by the almighty power of God on purpose to convince them that they had done very wickedly in asking a king; not only by its coming in an unusual time, in wheat-harvest, and this on a fair clear day, when there appeared not to the eye any signs of a storm, but by his giving notice of it before.

Now therefore stand,....

This great thing - This unusual occurrence.