Blessed Alan de la Roche, who was so deeply devoted to the Blessed Virgin, had many revelations from her, and we know that he confirmed the truth of these revelations by a solemn oath.
Three of them stand out with special emphasis: the first, that if people fail to say the Hail Mary, which has saved the world, out of carelessness, or because they are lukewarm, or because they hate it, this is an indication that they will probably be condemned to eternal punishment.
The second truth is that those who love this divine salutation bear the very special stamp of predestination.
The third is that those to whom God has given this favor of loving our Lady and of serving her out of love must take very great care to continue to love and serve her until the time when she shall have had them placed in heaven by her Son in the degree of glory which they have earned (Blessed Alan)
Heretics, all of whom are children of the devil and who clearly bear the sign of God's reprobation, have a horror of the Hail Mary. They still say the Our Father, but never the Hail Mary; they would rather carry a poisonous snake about them than a rosary.
Among Catholics, those who bear the mark of God's reprobation think but little of the Rosary. They either neglect to say it or only say it quickly and in a lukewarm manner.
Even if I did not believe what was revealed to Blessed Alan de la Roche, even then my own experience would be enough to convince me of this terrible but consoling truth. I do not know, nor do I see clearly, how it can be that a devotion which seems to be so small can be the infallible sign of eternal salvation, and how its absence can be the sign of God's eternal displeasure; nevertheless, nothing could be more true.
In our own day we see that people who hold new doctrines that have been condemned by the Church, with all their would-be piety, ignore the devotion to the Rosary and often dissuade their acquaintances from saying it with all sorts of fine pretexts. They are very careful not to condemn the Rosary and the Scapular, as the Calvinists do, but the way they set about attacking them is all the more deadly because it is the more cunning. I shall refer to it again later on.
The Hail Mary, the Rosary, is the prayer and the infallible touchstone by which I can tell those who are led by the Spirit of God from those who are deceived by the devil. I have known souls who seemed to soar like eagles to the heights by their sublime contemplation and yet were pitifully led astray by the devil. I only found out how wrong they were when I learned that they scorned the Hail Mary and the Rosary, which they considered as being far beneath them.
The Hail Mary is a blessed dew that falls from heaven upon the souls of the predestinate. It gives them a marvelous spiritual fertility so that they can grow in all virtues. The more the garden of the soul is watered by this prayer, the more enlightened in mind we become, the more zealous in heart, the stronger against all our enemies.
The Hail Mary is a sharp and flaming shaft which, joined to the Word of God, gives the preacher the strength to pierce, move, and convert the most hardened hearts, even if he has little or no natural gift for preaching.
As I have already said, this was the great secret that our Lady taught St. Dominic and Blessed Alan for the conversion of heretics and sinners. Saint Antoninus tells us that that is why many priests acquired the habit of saying a Hail Mary at the beginning of their sermons.
(Secret of the Rosary, by Saint Louis de Montfort)