" I have just said that to say the Rosary to advantage one must be in a state of grace “or at least be fully determined to give up mortal sin;” first of all, because, if it were true that God only heard the prayers of those in a state of grace it would follow that people in a state of mortal sin should not pray at all. This is an erroneous teaching which has been condemned by Holy Mother Church, because of course sinners need to pray far more than good people do. Were this horrible doctrine true it would then be useless and futile to tell a sinner to say all, or even part of his Rosary, because it would never help him. "
― Louis de Montfort , The Saint Louis de Montfort Collection [7 Books]