By Saint Louis de Montfort, Doctor of The Church, great luminary of St. John Paul II..
“If you say the Rosary faithfully until death, I do assure you that, in spite of the gravity of your sins “you shall receive a never-fading crown of glory” (1 Pet. 5:4). Even if you are on the brink of damnation, even if you have one foot in hell, even if you have sold your soul to the devil as sorcerers do who practice black magic, and even if you are a heretic as obstinate as a devil, sooner or later you will be converted and will amend your life and save your soul, if—and mark well what I say—if you say the Rosary devoutly every day until death for the purpose of knowing the truth and obtaining contrition and pardon for your sins.
In this book there are several stories of great sinners who were converted through the power of the Rosary. Please read and meditate upon them.” — Saint Louis de Montfort, The Secret of the Rosary
— See also A Letter to the Friends of the Cross by St. Louis de Montfort