"He must say many rosaries first..." - Our Lady of Fatima
"The sinner who has lost sanctifying grace cannot merit in this state, for sanctifying grace is the radical principle of all supernatural merit. Yet, by an actual transitory grace, the sinner can pray; he can ask the grace of conversion; and if he asks for it with humility, confidence, and perseverance, he will obtain it. Whereas merit, which is a right to a reward, is related to divine justice, prayer is addressed to the mercy of God, which often restores fallen souls and hears their prayers without any merit on their part.
... Moreover, the just man may obtain by prayer certain graces which he could not merit, in particular the gift of final perseverance. This gift cannot be merited, for it is nothing other than the continuation until death of the state of grace, which is the principle of merit. Obviously it would be impossible to merit the very principle of merit. However, final perseverance or the grace of a happy death can be obtained by humble, trusting, daily prayer.
For this reason the Church invites us to say daily with fervor in the second part of the Hail Mary; 'Holy Mary, Mother of God, pray for us sinners now and at the hour of our death . Amen.' Here prayer goes farther than merit, addressing itself, not to divine justice but to infinite mercy." - Three Ages of the Interior Life; Vol. I, Chapter VII: The Increase of the Life of Grace Through Prayer
Fifteen Promises of the Holy Rosary
Our Lady of the Rosary of Santiago