Well I didn't make this up, I read it from St. Louis de Montfort:
To say the Holy Rosary to advantage one must be in a state of grace or at the very least be fully determined to give up mortal sin. This we know because all our theology teaches us that good works and prayers are only dead works if they are done in a state of mortal sin. Therefore they can neither be pleasing to God nor help us gain eternal life. This is why Ecclesiastes says: “Praise is not seemly in the mouth of a sinner.” [1] Praise of God and the salutation of the Angel and the very Prayer of Jesus Christ are not pleasing to God when they are said by unrepentant sinners.
Our Lord said: “This people honoreth Me with their lips, but their heart is far from Me.” [2] It is as though He was saying: “Those who join My Confraternity and say their Rosary every day (even perhaps the fifteen decades), but without being sorry for their sins offer Me lip service only and their hearts are far from Me.”
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.
Secondly, because if they join one of Our Lady’s confraternities and recite the Rosary or some other prayer, but without having the slightest intention of giving up sin, they join the ranks of her false devotees. These presumptuous and impenitent devotees, hiding under her mantle, wearing the scapular and with rosary in hand, cry out: “Blessed Virgin, good Mother—-Hail, Mary! . . .” And yet at the same time, by their sins, they are crucifying Our Lord Jesus Christ and tearing His flesh anew. It is a great tragedy, but from the very ranks of Our Lady’s most holy Confraternities souls are faIling into the fires of Hell.
We earnestly beg everyone to say the Holy Rosary: the just that they may persevere and grow in God’s grace; the sinners that they may rise from their sins. But God forbid that we should ever encourage a sinner to think that Our Lady will protect him with Her mantle if he continues to love sin, for then it will only turn into a mantle of damnation which will hide his sins from the public eye. The Rosary, which is a cure for all our ills, would then be turned into deadly poison. “A corruption of what is best is worst.”
The learned Cardinal Hughes says: “One should really be as pure as an Angel to approach the Blessed Virgin and to say the Angelic Salutation.” One day Our Lady appeared to an immoral man who used to always say his Rosary every day. She showed him a bowl of beautiful fruit, but the bowl itself was covered with filth. The man was horrified to see this, and Our Lady said: “This is the way you are honoring me! You are giving me beautiful roses in a filthy bowl. Do you think I can accept presents of this kind?”