John Calvin Commentary Exodus 15:14

"The peoples have heard, they tremble: Pangs have taken hold on the inhabitants of Philistia." — Exodus 15:14 (ASV)

The people shall hear. Again, in this place, I have not hesitated to change the tenses. It is clear that Moses is speaking of future events. However, I do not deny that by using past tense verbs, he confirms the certainty of the matter—a common figure of speech among the Prophets.

This boast depends on the mention of God’s “strength.” For it would have been impossible for the Israelites to make their way through so many hostile nations into the land of Canaan unless God had, so to speak, extended His hand from heaven and fought for them. Therefore, to prevent their numerous difficulties from disheartening them, Moses declares that even if many powerful enemies attempt to oppose them, terror from heaven shall possess them all, so that, in their confusion and astonishment, they shall have no power to resist.