Why the Rosary is "a short version of the Gospel"

"Great is the strength of an army that holds in its hand not the sword but the Rosary." 

These words of Blessed Pope Pius IX (d. 1878) are enough to refute the argument that the Rosary, because of its repetitive aspect, is a mechanical, superficial or ineffective prayer. Those who are willing to use this prayer called 'prayer of the simple' are likely to experience, in a tangible way, its extraordinary effectiveness. Many popes have encouraged this devotion, and, thanks especially to Marian apparitions that have constantly urged its recitation, it has withstood the test of time, spanning centuries, and is enjoying renewed popularity today.

In its present form, the Rosary is composed of 20 mysteries, helping us to look more closely at the life of Jesus: this is why it is dubbed a "short version of the gospel."

The use of the word "mystery" deserves a little digression: etymologically, this word designates the profound character, the inherent virtue of a thing, a secret within the Christian faith: the sacrifice of the Mass is thus called the Holy Mystery. Medieval theatrical presentation used the term for the staging of the Nativity, the Passion, the Resurrection of Christ and the lives of the saints. It is in this sense that we speak of the "mysteries'' of the Rosary.

The full Rosary used to include three 5-decade rosaries, corresponding to three series of "mysteries" of the life of Christ and the Virgin Mary: 

The Joyful mysteries of Jesus' childhood (Annunciation of Jesus' birth to Mary, Visitation of Mary to Elizabeth, Nativity, Presentation of Jesus in the Temple of Jerusalem, Finding of Jesus after he was lost during a pilgrimage to Jerusalem); the Sorrowful mysteries of Jesus' Passion (Agony, Scourging, Crowning with thorns, Carrying of the Cross, Crucifixion), and the Glorious mysteries (Resurrection, Ascension, Pentecost, Assumption, Crowning of Mary). 

Reviving a medieval tradition, Pope John Paul II added a fourth series: the Luminous mysteries, corresponding to the public life of Christ, extending from the Baptism of Jesus to just before his Passion (Baptism, Wedding at Cana, Proclamation of the Kingdom, Transfiguration, Institution of the Holy Eucharist).

The 5-decade rosary is a short form of the full Rosary, and consists of just 5 mysteries. Those who pray 5 decades every day recite the joyful mysteries on Monday and Saturday, the sorrowful mysteries on Tuesday and Friday, and the glorious mysteries on Wednesday and Sunday, with Thursday now dedicated to the luminous mysteries.

Isabelle Rolland, excerpt from Le Rosaire, présent du Ciel et chemin de sainteté (The Rosary, Gift from Heaven and Path to Holiness)