John Calvin Commentary Acts 13:9

"But Saul, who is also [called] Paul, filled with the Holy Spirit, fastened his eyes on him," — Acts 13:9 (ASV)

And Saul, who was also called Paul. Luke now shows how God broke the bond in which the deputy was bound. For since he was too devoted to the magician, he could not embrace true doctrine as one who was free and at liberty. For the devil keeps those minds (which he has entangled) in his slavery in such an astonishing and incredible manner that they cannot see even the plainest truth. But as soon as the magician was vanquished, Paul could easily gain access to the deputy.

And note what Luke says: that the faith is overthrown when the word of God is resisted. From this we may gather that faith is so grounded in the word that without this foundation, it falters at every assault; indeed, that it is nothing else but the spiritual building of the word of God.