The people had just walked through the sea on dry ground, and now they sing of what held them safe. Behind them the great powers that terrified them stand frozen, still as a stone. A raging force is not stopped by more noise or a bigger army but by a stillness that reaches from beyond any threat.
The same thing happens in us. When the mind grows quiet, the old fears that once thundered lose their motion and cannot follow. That silence is called here the greatness of thine arm, a strength without strain. Held by it, a person passes safely through what looked impassable, and the panic that chased them simply goes quiet.