Since the eight century the feast of the conversion of St. Paul has been set apart by the Church to return thanks to almighty God for His act of grace in bringing the future apostle to the Faith.  At on time the feast was even a holy day of obligation.  After the miracle of Christ’s Resurrection no other wonder in the history of the early Church is a stronger proof of the divine origin of Christianity than the conversion of St. Paul.

(Source: Fr. Lasance, The New Roman Missal)