This miraculous way in which the devotion to the Holy Rosary was established is something of a parallel to the way in which Almighty God gave His law to the world on Mount Sinai and obviously proves its value and importance.
Inspired by the Holy Ghost, instructed by the Blessed Virgin as well by his own experience, Saint Dominic preached the Holy Rosary for the rest of his life. He preached it by his example as well as by his sermons, in cities in country places, to people of high station and low, before scholars and the uneducated, to Catholics and to heretics.
The Holy Rosary which he said every day was his preparation for every sermon and his little tryst with Our Lady immediately after preaching.
One Day he had to preach at Notre Dame in Paris, and it happened to be the feast of St. John the Evangelist. He was in a little chapel behind the high altar prayerfully preparing his sermon by saying the Rosary, as he always did, when Our Lady appeared to him and said:
“Dominic, even though what you have planned to say may be very good, I am brining you a much better sermon.”
Saint Dominic took in his hands the book Our Lady proffered, read the sermon carefully and when he had understood it and meditated on it, he gave thanks to the Blessed Mother.
When the time came, he went up into the pulpit and, in spite of the feast day, made no mention of Saint John other than to say that he had been found worthy to be the guardian of the Queen of Heaven. The congregation was made up of theologians and other eminent people who were used to hearing unusual and polished discourses; but Saint Dominic told them that it was not his wish to give them a learned discourse, wise in the eyes of the world, but that he would speak in the simplicity of the Holy Spirit and with His forcefulness.
So he began preaching the Holy Rosary and explained the Hail Mary word by word as he would to a group of children and used the very simple illustrations which were in the book Our Lady had given to him.
Carthagena, the great scholar, quoting Blessed Alan de la Roche in “De Dignitate Psalterii,” describes how this took place:
“Blessed Alan writes that one day Father Dominic said to him in a vision: ‘My son, it is good to preach; but there is always a danger of looking for praise rather than the salvation of souls. Listen carefully to what happened to me in Paris so that you may be on guard against this kind of mistake: I was to preach in the great church dedicated to the Blessed Virgin Mary and I was particularly anxious to give a brilliant sermon, not out of pride, but because of the high intellectual stature of the congregation.
‘An hour before the time I had to preach, I was recollectedly saying my Rosary – as I always did before giving a sermon – when I fell into ecstasy. I saw my beloved friend the Mother of God coming towards me with a book in her hand.
‘“Dominic,” she said, “‘your sermon for today may be very good indeed, but no matter how good it is I have brought you one that is very much better.”
‘Of course I was overjoyed, took the book and read every word of it. Just as Our Lady had said, I found exactly the right things to say in my sermon, so I thanked her with all my heart.
‘When it was time to begin, I saw that the University of Paris had turned out in full force as well as a large number of noblemen. They had all seen and heard of the great things that the good Lord had been doing through me. So I went up into the pulpit.
‘It was the feast of Saint John the Apostle but all I said about him was that he had been found worthy to be the guardian of the Queen of Heaven. Then I addressed the congregation:
“‘My Lords and illustrious Doctors of the University, you are accustomed to hearing learned sermons suited to your aesthetic tastes. Now I do not want to speak to you in the scholarly language of human wisdom but, on the contrary, to show you the Spirit of God and His Greatness.'”
Here ends the quotation from Blessed Alan, after which Carthagena goes on to say in his own words:
“Then Saint Dominic explained the Angelic Salutation to them, using simple comparisons and examples from everyday life.”
Blessed Alan, according to Carthagena, mentioned several other times when Our Lord and Our Lady appeared to Saint Dominic to urge and inspire him to preach the Rosary more and more in order to wipe out sin and to convert sinners and heretics.
In another passage Cathagena says:
“Blessed Alan said Our Lady revealed to him that after she had appeared to Saint Dominic, her Blessed Son appeared to him and said:
‘Dominic, I rejoice to see that you are not relying upon your own wisdom and that, rather than seek the empty praise of men, you are working with great humility for the salvation of souls.
‘But many priests want to preach thunderously against the worst kinds of sin at the very outset, failing to realize that before a sick person is given bitter medicine he needs to be prepared by being put in the right frame of mind to really benefit by it.
‘This is why, before doing anything else, priests should try to kindle a love of prayer in people’s hearts and especially a love of my Angelic Psalter. If only they would all start saying it and would really persevere, God, in His mercy, could hardly refuse to give them His grace. So I want you to preach my Rosary.’
In another place Blessed Alan says:
“All priests say a Hail Mary with the faithful before preaching, to ask for God’s grace. They do this because of a revelation that Saint Dominic had from Our Lady. ‘My son,’ she said one day ‘do not be surprised that your sermons fail to bear the results you had hoped for. You are trying to cultivate a piece of ground which has not had any rain. Now when Almighty God planned to renew the face of the earth He started by sending down rain from heaven – and this was the Angelic Salutation. In this way God made over the world.
‘So when you give a sermon, urge people to say my Rosary, and in this way your words will bear much fruit for souls.’
“Saint Dominic lost no time in obeying, and from then on he exerted great influence by his sermons.”
This last quotation is from the “Book of Miracles of the Holy Rosary” (written in Italian) and it is also to be found in Justin’s works (143d Sermon).
I have been very glad to quote these well-known authors word for word in the original Latin  for benefit of any priests or other learned people who might otherwise have doubts as to the marvelous power of the Holy Rosary.
As long as priests followed Saint Dominic’s example and preached devotion to the Holy Rosary, piety and fervor thrived throughout the Christian world and in those religious orders which were devoted to the Rosary. But since people have neglected this gift from heaven, all kinds of sin and disorder have spread far and wide.
 We have omitted the Latin quotations so as not to encumber the text. English translations have been given above.