More importantly, for Toplady’s verses, the water flowing from the rock was a type or foreshadowing of the water that flowed, together with blood, from the side of Christ when he was pierced by a spear as he hung on the cross.
Toplady and his congregation were equally aware of the water that flowed from the right side of the temple in the vision of Ezekiel (47:1). That verse is sung round the world at Eastertide (and has been set by great composers such as Victoria) during the Asperges, the ritual sprinkling of the people at the beginning of Mass. That is not a practice of which Toplady would have approved, although the biblical reference is the same.
And this is what is so strange about Toplady’s devotion to the wounds of Christ, the real subject of his hymn. They (standing for Christ’s one sacrifice in his suffering and death) have saving power. The hymn writer wants to “hide himself” in them – at face value a grisly desire. Yet it is one that medieval mystics expressed too – Julian of Norwich springs to mind.