St. Maximilian Kolbe called the Miraculous Medal his "bullet" that he used as a spiritual weapon.
Many saints had their favorite prayer asking for God’s grace to convert a wayward soul. For St. Maximilian Kolbe, the Miraculous Medal was his spiritual weapon of choice.
Kolbe became convinced of the Miraculous Medal’s spiritual influence when he heard the story of Alphonse Ratisbonne.
Ratisbonne was given a Miraculous Medal from a Catholic convert. While at first Ratisbonne protested at wearing the medal, he decided to put it around his neck and say the Memorare each day. He figured that it couldn’t do any harm and would prove to all the ridiculous nature of Catholicism.
One day when Ratisbonne entered a 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 and converted to the Catholic faith.
St. Maximilian Kolbe heard this story and it became a primary impetus behind his own use of the Miraculous Medal.
He wrote in a letter how he viewed the use of the Miraculous Medal.
The Miraculous Medal should be the weapon, the bullet, which the Knight of the Immaculata makes use of. Even if someone is as wicked as can be, if he agrees to wear the Miraculous Medal, give it to him and pray for him, and occasionally try with a kind word to bring him to the point where he begins to love the Mother of God and to fly to her in all his difficulties and temptations. But anyone who sincerely begins to pray to the Immaculata will soon be convinced to go to Confession as well. There is much evil in the world, yet let us consider that the Immaculata is even more powerful.
The Miraculous Medal was not only distributed by Kolbe, but was also worn by those in his religious order and in the Milita Immaculatae, the group of spiritual disciples who followed Kolbe’s teachings.
The Medal itself does not possess any power, as it is a sacramental in the Catholic Church. This means that it is an extension of the seven sacraments and invokes the grace of God.
As with any sacramental, miracles of conversion occur through God’s providential plan, inspiring faith in Jesus Christ. All credit goes back to God, who can use various material things to bring souls back to him.