Struck once in Exodus, spoken to in Numbers. Paul says that Rock was Christ, and the change in method is one of the clearest pictures of the cross in the Old Testament.
The Shadow
Twice Israel ran out of water, and twice God brought it from a rock, but the instructions were different. In Exodus 17: strike the rock with the rod of judgment, the same rod that struck the Nile. In Numbers 20: speak to the rock. Moses, angry, struck it twice instead, and the consequence was staggering. The man who led Israel out of Egypt would never enter the land. Most sermons make that a lesson about obedience. But Moses did not merely lose his temper. He shattered a picture God was painting of something that had not happened yet.
The Fulfillment
Paul removes all ambiguity: they drank of that spiritual Rock that followed them, and that Rock was Christ. The rock was struck first because Christ would be struck. Judgment came down on the innocent so provision could flow to the guilty: living water from a broken body. And the second instruction changed because once Christ has been struck, He never has to be struck again. Hebrews seals it: after He had offered one sacrifice for sins forever, He sat down. The striking is over. Now the provision reaches you by a word, believing in your heart and confessing with your mouth, not by the rod.
That is why Moses's error cost so much. Striking the rock a second time preached the wrong covenant to a watching nation. And Moses, the law's own representative, could not enter the land, because the law can lead you out of Egypt but cannot carry you into rest. Only Joshua, whose Hebrew name is the same as Jesus, does that.
Him All Along
If your spiritual life runs on a striking rhythm, sin, punish yourself, provision, repeat, hear the rock's message: the striking is finished. Christ was struck once, and every drop of provision you will ever need came out of that unrepeatable moment. The rock is already open. The water has not stopped flowing. Put the rod down.