St. Alphonsus de Liguori, founder of the Redemptorists and Doctor of the Church, has this to say about mental prayer:
“Prayer is absolutely necessary for our salvation, and mental prayer is necessary to pray well. Without mental prayer we can have no supernatural light concerning God, the eternal truths, the mysteries of faith, the virtues and vices, the vanity of earthly goods, honors and pleasures, and the grandeur and excellence of the spiritual and heavenly goods. Without it we cannot acquire the necessary knowledge of ourselves, our misery, our helplessness, nor can we discover our faults or the means of avoiding and correcting them. Hence we cannot pray with sufficient earnestness, humility, confidence and perseverance for light and strength to avoid sin and perform our duties, or to overcome temptations or to subdue our passions, unless we practice mental prayer. ‘The earth is laid waste, because there is no one that thinketh in his heart’ (Jer 12:11).”
“Mental prayer excites and increases our faith and hope in God, enlightens us, moves our heart to love God, and strengthens our will in the path of duty, for God becomes therein our teacher: ‘Blessed is the man, whom Thou, O Lord, shalt instruct’ (Ps 93:12). Mental prayer renders us watchful over ourselves, mistrustful of self and full of confidence in God. He who daily makes mental prayer discovers the wants of his soul, and learns how to pray to God to grant them. Moreover, like a fiery furnace, it heats and softens the cold and hard iron of our heart, and renders it malleable in the hands of God.”
“He who faithfully practices mental prayer and perseveres in it cannot be lost, for mortal sin and mental prayer cannot abide together. A sinner who begins to make mental prayer will soon be compelled either to give up sin altogether, or to cease making mental prayer; and, however deeply rooted his habits may be, if he persists in this holy exercise, it will soon enable him to make all the sacrifices necessary to give up the habit of sin entirely. In mental prayer the soul is filled with holy thoughts, holy affections, desires and resolutions and with divine love and thereby strengthened to give up every evil habit and sacrifice all her passions.”
Quotations from Ferreol Girardey, Prayer: Its Necessity, Its Power, Its Conditions (St. Louis: B. Herder, 1916).