Union with God is the paramount goal of religion for some Catholics. Catholics believe that God seeks unification with His children - Mankind. Prayer is the expression of the soul seeking to speak with God. St. Therese of the Child Jesus explains: "For me, prayer is an uplifting of the heart, a glance toward heaven, a cry of gratitude and of love in times of sorrow as well as of joy" (Story of a Soul). Catholics seek to elevate the heart and mind to God. Prayer, as discourse with a friend, is not constrained, but rather is spontaneous. Certain prayer events are organized, of course, as in the case of the Breviary, or the Mass, or other liturgical events; yet even during such events the specific discourse between the soul and God may be spontaneous. A Psalm, for example, may offer different meanings depending on the mood of the person praying. For the Catholic, prayer is love, and to "Pray always" (Lk 18:1) is to love always.
The Catholic grows spiritually through the life of prayer. A classic way to distinguish among phases of growth is three-fold: beginners start on the "purgative way", later comes the "illuminative way" with "affective prayer", and eventually one may experience the "unitive way". Catholics who have been especially helpful in developing an understanding of prayer include St. John of the Cross and St. Teresa of Avila, both Doctors of the Roman Catholic Church. In the purgative way, the Catholic attempts to leave behind a life of sin, for "sin is iniquity" (Jn 3:4). The Catholic is enjoined to recall that "my sin is always before me" (Psalms 51:5). Growth toward holiness is ongoing, as the Apostle writes "he that is holy let him be sanctified still" (Apoc 22:11).
Next, in the illuminative way, the soul seeks the imitation of Christ to "have the light of life" (Jn 8:12). There is a famous book by Thomas à Kempis titled The Imitation of Christ. While in purgative prayer one is mostly engaged in vocal prayer, in the illuminative way one tends to be more affective along the lines suggested by St. Therese, above. Affective prayer of this sort may best permit us to fulfill the command "always to pray and not to faint" (Lk 18:1). Last, in the unitive way the soul now seeks to say "I live now not I; but Christ livith in me!" (Gal 2:20). For Roman Catholics, the Eucharist is a key expression of this union with Jesus.