18 “They forgat God who saved them”. How did He save them? “Who did so great things in Egypt: Wondrous works in the land of Ham, and fearful things in the Red Sea”. The things that are wondrous, are also fearful; for there is no wonder without a certain fear: although these might be called fearful, because they beat down their adversaries, and showed them what they ought to fear.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)