Has a notable militant non-Christian ever converted to Christianity due to a “Road to Damascus” experience?
I suppose in the history of the Church, such things have happened. The question now is where to find the sources.
All will depend on how one interpreted the phrase: a “Road to Damascus” experience?
To start with I will begin with an Early Church Martyr.
Martyr Evilasius (died 311) was a pagan priest who tortured a 13-year-old girl who later became Saint Fausta. Realizing her courage, he himself converted to Christianity, an act punishable by death since the people of Cyzicus did not want even one of their people to convert to any religion other than their own.
If by a "Road to Damascus" you mean a case where someone is converted wholly by a supernatural vision of some kind, then Marie-Alphonse Ratisbonne should fit the bill.
Alphonse Ratisbonne was a staunch French atheistic Jew who was miraculously converted while at Rome in the Church of San Andrea della Frate, on January 20, 1842, following a vision of the Blessed Virgin Mary. His brother had previously converted and become a Catholic priest, but Alphonse hated the Catholic Church and vowed never to enter.
One day when visiting his friend Ratisbonne encountered a Catholic convert, Theodore de Bussieres, who knew Ratisbonne’s priest-brother. While this made Ratisbonne hate the man, he enjoyed conversing with him because of his knowledge.
Later Ratisbonne visited de Bussieres again. They had a heated discussion about Catholicism and de Bussieres made a wager with Ratisbonne.
Have you the courage to submit yourself to a very simple and innocent test? Only to wear a little something I will give you; look, it is a medal of the Blessed Virgin. It seems very ridiculous, does it not? But, I assure you, I attach great value and efficacy to this little medal. [Also] you must say every night and morning the Memorare, a very short and very efficacious prayer which St. Bernard addressed to the Blessed Virgin Mary.
While at first Ratisbonne protested at wearing the medal (which was the Miraculous Medal), he decided to put it around his neck and say the prayer each day. He figured that it couldn’t do any harm and would prove to all the ridiculous nature of Catholicism.
Ratisbonne lived up to his side of the bargain, finding it easy to recite the Memorare. Then one day he was traveling in the city with de Bussieres and they stopped at the church Saint Andrea delle Fratte. When Ratisbonne entered the church it appeared to be engulfed in a marvelous light. He looked to an altar from where the light was coming and saw the Virgin Mary, appearing as she did on the Miraculous Medal.
He left the church in tears, clutching his Miraculous Medal. Several days later he was received into the Catholic Church. After returning to Paris his betrothed was shocked and rejected him and his new religion. Ratisbonne then entered the Jesuits and was ordained a priest.
This amazing story of conversion would later influence Saint Maximilian Kolbe to found the Militia Immaculatae and convinced him of the power of the Miraculous Medal. He firmly believed in Mary’s role in bringing the world to Christ. - How a radical atheist became a Catholic priest
The Miraculous Medal played an important role in the Conversion of Alphonse Ratisbonne
The apparition to Alphonse Ratisbonne in Rome approved by the Holy See (Pope Pius IX) on June 3, 1842.
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