Zeal of the convert
The zeal of the convert is a term describing particularly fervent devotion to new beliefs at variance to one's old beliefs. For example, Paul the Apostle, formerly known as Saul of Tarsus, was a Jewish Pharisee who persecuted Christians until he had a life changing vision on the road to Damascus and became a missionary who spent his life spreading the religion throughout the Roman empire and was executed in Rome for his beliefs.