John Calvin Commentary Acts 13:7

"who was with the proconsul, Sergius Paulus, a man of understanding. The same called unto him Barnabas and Saul, and sought to hear the word of God." — Acts 13:7 (ASV)

And since Sergius Paulus, desiring something better than what he had learned from his childhood, was inappropriately drawn aside to various superstitions, I gather from this that he sends for Paul and Barnabas on his own initiative, to teach him. Therefore, he had formed a certain reverence and fear of God, though he did not yet know Him; and because he was persuaded that the God worshipped in Judea was the true God, he desired to learn from His word a pure and certain rule of godliness.

As soon as he experienced the delusions of the false prophet, he began to doubt. And undoubtedly, God prompts his mind so that he might not be entirely held back by vanity, even though he allowed himself to be deceived for a time by a wicked man.