John Calvin Commentary Psalms 135:8

"Who smote the first-born of Egypt, Both of man and beast;" — Psalms 135:8 (ASV)

He smote the first-born of Egypt. He now returns to those more special benefits by which God had placed His Church and chosen people under obligation to His service. Since it was only the Lord’s believing people whom He addressed, the chief point singled out as the subject of praise is God’s having adopted them, though they were small in number, from the mass of humanity.

Again, there was the fact of His having set Himself in opposition, for their sake, to great kingdoms and mighty nations. The wonderful works done by God in Egypt and in Canaan were all just so many proofs of that fatherly love which He had for them as His chosen people. It is not strictly according to historical order to begin by mentioning the destruction of the first-born of Egypt; but this is cited as a memorable illustration of the great regard God had for the safety of His people—so great that He would not spare even such a mighty and wealthy nation. The purpose of the passage is to show that God, in delivering His people, had abundantly testified to His power and His mercy.