According to St. Louis de Montfort:

"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.” 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 Holy Rosary devoutly every day until death for the purpose of knowing the truth and obtaining contrition and pardon for your sins."

https://www.ecatholic2000.com/montfort/rosary/rosary.shtml