Mary to Saint Dominic: The Holy Trinity willed to use the weapon of the Rosary to reform the world

It was only in the year 1214, however, that the Church received the Rosary in its present form and according to the method we use today. It was given to the Church by St. Dominic, who had received it from the Blessed Virgin as a means of converting the Albigensians and other sinners. 

I will tell you the story of how he received it, which is found in the very well-known book De Dignitate Psalterii, by Blessed Alan de la Roche. Saint Dominic, seeing that the gravity of people's sins was hindering  the conversion of the Albigensians, withdrew into a forest near Toulouse, where he prayed continuously for  three days and three nights. During this time he did nothing but weep and do harsh penances in order to  appease the anger of God. He used his discipline so much that his body was lacerated, and finally he fell into a coma. 

At this point our Lady appeared to him, accompanied by three angels, and she said, "Dear Dominic, do you know which weapon the Blessed Trinity wants to use to reform the world?" 

"Oh, my Lady," answered Saint Dominic, "you know far better than I do, because next to your Son Jesus Christ you have always been the chief instrument of our salvation." 

Then our Lady replied, "I want you to know that, in this kind of warfare, the principal weapon has always been the Angelic Psalter, which is the foundation-stone of the New Testament. Therefore, if you want to reach these hardened souls and win them over to God, preach my Psalter." 

Thus was born the Most Holy Rosary of Mary, which St. Dominic immediately employed against the Albigensian heresy. Blanche of Castile immediately embraced this new devotion, to which she attributed the birth of her son, the future king Saint Louis.

Saint Louis de Montfort, The Secret of the Rosary, Second Rose