Acts 13:12

Then the proconsul, when he saw what was done, believed, being astonished at the teaching of the Lord.

Believed ... As frequently, especially in Paul's writings, this is a synecdoche, a type of metaphor in which one of a related group of actions stands for all of them. The meaning here is that Paulus believed the gospel of Christ, repented of his sins, confessed the Saviour, and was baptized into Christ, becoming a Christian. Luke used this same figure in Acts 16:34, in which place, after spelling out certain preconditions of salvation fulfilled by the jailer, he spoke of the jailer's compliance with all of them as his "having believed in God."

Paul's miracle had the intended effect. It fully convinced Paulus and inflicted a severe judgment upon Elymas, but in such a manner as to leave him opportunity for repentance. Those who attempt to find in this event some grounds of disapproval for Paul's actions have simply failed to read it correctly.