EVER SINCE Saint Dominic established the devotion to the Holy Rosary up until the time when Blessed Alan de la Roche re-established it in 1460, it has always been called the Psalter of Jesus and Mary. This is because it has the same number of Angelic Salutations as there are Psalms in the Book of the Psalms of David. [Emphasis added.] Since simple and uneducated people are not able to say the Psalms of David the Rosary is held to be just as fruitful for them as David's Psalter is for others.
But the Rosary can be considered to be even more valuable than the latter for three reasons:
1. Firstly, because the Angelic Psalter bears a nobler fruit, that of the Word Incarnate, whereas David's Psalter only prophesies His coming;
2. Secondly, just as the real thing is more important than its prefiguration and as the body is more than its shadow, in the same way the Psalter of Our Lady is greater than David's Psalter which did no more than prefigure it;
3. And thirdly, because Our Lady's Psalter (or the Rosary made up of the Our Father and Hail Mary) is the direct work of the Most Blessed Trinity and was not made through a human instrument.
Our Lady's Psalter or Rosary is divided up into three parts of five decades each, for the following special reasons:
1. To honor the three Persons of the Most Blessed Trinity;
2. To honor the life, death and glory of Jesus Christ;
3. To imitate the Church Triumphant, to help the members of the Church Militant and to lessen the pains of the Church Suffering;
4. to imitate the three groups into which the Psalms are divided:
(a) the first being for the purgative life,
(b) the second for the illuminative life,
(c) and the third for the unitive life;
5. And finally, to give us graces in abundance during our lifetime, peace at death, and glory in eternity.
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Nihil Obstat, Gulielmus F. Hughes, S.T.L., Censor Librorium
Imprimatur, Thomas Edmundus Molloy, S.T.D., Archiepiscopus-Episcopus Brooklyniensis, 1954 For more, see The Holy Rosary
St. Louis-Marie Grignon de Montfort's The Secret of the Rosary