﴿ لَوْ نَشَآءُ جَعَلْنَٰهُ أُجَاجًۭا فَلَوْلَا تَشْكُرُونَ ﴾

“[It comes down sweet - but] were it Our will, We could make it burningly salty and bitter: why, then, do you not give thanks [unto Us]?”

(If We willed We verily could make it) i.e. pure water (bitter. Why, then, give ye not thanks) for its purity and therefore believe?