St. Josemaria received his First Communion on April 23, 1912, a day he would fondly remember. He began a lifelong, intimate relationship with Jesus in the Eucharist on that day.
Prior to that fateful day, St. Josemaria was prepared for his First Communion by a priest named Fr. Manuel Laborda de la Virgin del Carmen. He taught little Josemaria a short little prayer to help prepare his soul.
It is a prayer that did not originate with Fr. Manuel, but from a 19th century catechism.
According to Fr. Jesus Sancho, in an article on OpusDei.org, the prayer originated in a catechism "By Father Cayetano de San Juan Bautista. Priest of said Piarist Schools. It was printed in Pamplona in the year 1800 and has 357 pages. I began reading it little by little, and great was my surprise to come upon the following prayer, on page 308, where the author is seeking to strengthen the reader’s desire to receive our Lord in Holy Communion."
St. Josemaria would continue to say this prayer throughout his life, finding peace within it and strengthening his faith in the Eucharist.