John Gill Commentary Psalms 97:4

"His lightnings lightened the world: The earth saw, and trembled." — Psalms 97:4 (ASV)

His lightnings enlightened the world
Either the doctrines of the Gospel, compared thereunto, because of the swift progress they made, and the large extent of them in the world, in a very little time; by the apostles they were published in all nations, and were the means of enlightening them in the true knowledge of themselves, and of the way of salvation by Christ: hence they are called the "lights of the world", (Matthew 5:14) , as the coming of Christ, in his kingdom and power, by them, is compared to lightning, and so are the arrows of his word, (Matthew 24:27) (Zechariah 9:14) , or else his judgments on the Jewish nation are meant, which were manifest and clear, and obvious to all the world; see (Psalms 18:14) ,

the earth saw, and trembled;
the inhabitants of the earth, of the Gentile world, saw the judgments of God upon the Jews, and were astonished at them; see (Deuteronomy 29:24) , it is usual for lightnings and earthquakes to go together; see (Revelation 11:19) (16:15) .