John Calvin Commentary Acts 13:12

"Then the proconsul, when he saw what was done, believed, being astonished at the teaching of the Lord." — Acts 13:12 (ASV)

Then when the deputy saw This is what I said, that the snares in which Elymas kept him entangled were broken, for he was brought by the miracle to faith, because reverence for doctrine is the beginning of faith and its preparation. Therefore, since he saw clear evidence of the power of God, he knew that Paul was sent from God, and so he began to reverence his doctrine, which he had doubted before.

If God now miraculously strengthens in the minds of many the faith of the gospel, which is shaken by so many and such strong assaults; if He brings it about, in an incredible manner, that the course of faith passes through a thousand hindrances—then, being content with this His grace, let us not murmur against Him or argue with Him, as if our condition were worse, if He does not daily show such miracles as we would desire.