The pillar of cloud and fire led Moses and his people through the sea, out of Egypt, and into a stretch of country where it seemed they might die of hunger and thirst. When they complained of hunger, down from heaven came the manna; but then, instead of trusting that a God who could provide bread by a miracle could presumably provide water too, they resorted again to the complaints procedure.
So there at "the rock at Horeb" God told Moses, "Strike the rock, and water will come out of it" (Exod. 17:6). There is one thing very peculiar, but another thing very plain, about the apostle Paul's take on this incident. Whatever does he mean by "the rock that followed them"? Does he have in mind the Jewish legend of a rock that actually trundled round the desert with them like an affectionate pet? More likely he means that it was the Lord who never abandoned them; and the Lord, the "Rock," was Christ, even in Old Testament days.
But his plain point is unmissable. Neither an initial "baptism" nor continuing blessings were in themselves any guarantee that the hearts of these chronic complainers were right with God.
Thought: Gratitude, not grumbles - the sign of the true heart.