John Calvin Commentary Acts 9:4

"and he fell upon the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me?" — Acts 9:4 (ASV)

And therefore Luke says that he fell to the ground. For what else can happen to anyone but that they must lie prostrate and be, as it were, brought to nothing when they are overwhelmed with the immediate experience of God’s glory? And this was the beginning of Paul’s humbling, so that he might become prepared to hear the voice of Christ, which he had despised for so long while he sat proudly on his horse.

Saul, Saul! Luke compared the light which shone around Paul to lightning, though I do not doubt that lightning flashed in the air. And this voice, which Christ sent forth to crush his pride, may very well be called a lightning or thunderbolt, because it not only struck him and astonished him, but completely killed him, so that he was now as a nobody to himself—he who had previously been so pleased with himself and had claimed for himself the authority to suppress the gospel. Luke records his name in Hebrew here, Saul, Saul! because he is repeating the words of Christ, who undoubtedly spoke to him according to the common custom of the country.