“He who neglects mental prayer needs no devil to carry him to hell. He brings himself there with his own hands.” Yet “The devil knows that he has lost the soul that perseveringly practices mental prayer.” -Saint Teresa of Avila
-“It is morally impossible for him who neglects meditation to live without sin . . . Many say the Rosary, the Office of the Blessed Virgin, and perform other works of devotion; but they still continue in sin. But it is impossible for him who perseveres in mental prayer to continue in sin: he will either give up meditation or renounce sin.” -Saint Alphonsus
“Knowing how much is at stake, the devil wants at all costs to keep us from being faithful to mental prayer.” -Father Jacques Philippe
“To vocal prayer we must add mental prayer, which enlightens the mind, inflames the heart and disposes the soul to listen to the voice of wisdom, to savor his delights and possess his treasures. For myself, I know of no better way of establishing the kingdom of God, eternal wisdom, than to united vocal and mental prayer by saying the holy Rosary and meditating on its 15 mysteries.” – St. Louis de Monfort
“Mental prayer is the blessed furnace in which souls are inflamed with the love of God. All the saints have become saints by mental prayer.” -S Alphonsus
“In the first place, without mental prayer the soul is without light.They, says St. Augustine, who keep their eyes shut cannot see the way to their country. The eternal truths are all spiritual things that are seen, not with eyes of the body, but with the eyes of the mind; that is, by reflection and consideration. Now, they who do not make mental prayer do not see these truths, neither do they see the importance of eternal salvation, and the means which they can adopt in order to obtain it.
The loss of so many souls arises from the neglect of considering the great affair of our salvation, and what we must do in order to be saved. “With desolation,” says the prophet Jeremias, “is all the land made desolate: because there is none that considereth in the heart” (Jer 12:2). On the other hand, the Lord says that he who keeps before his eyes the truths of faith ——that is, death, judgment, and the happy or unhappy eternity that awaits us——shall never fall into sin. “In all thy works remember thy last end, and thou shalt never sin” (Sirach 7:36).
St. Bonaventure also says that mental prayer is, as it were, a mirror, in which we see all the stains of the soul. In a letter to the Bishop of Osma, St. Teresa says, “Although it appears to us that we have no imperfections, still when God opens the eyes of the soul, as He usually does in prayer, our imperfections are then clearly seen.”
He who does not make mental prayer does not even know his defects, and therefore, as St. Bernard says, he does not abhor them. He does not even know the dangers to which his eternal salvation is exposed, and, therefore, he does not even think of avoiding them. But he who applies himself to meditation instantly sees his faults, and the dangers of perdition, and, seeing them, he will reflect on the remedies for them. ” -St. Alphonsus: https://www.ourcatholicprayers.com/alphonsus-liguori-on-mental-prayer.html
“In mental prayer, the soul is purified from its sins, nourished with charity, confirmed in faith, and strengthened in hope; the mind expands, the affections dilate, the heart is purified, truth becomes evident; temptation is conquered, sadness dispelled; the senses are renovated; drooping powers revive; tepidity ceases; the rust of vices disappears. Out of mental prayer issues forth, like living sparks, those desires of heaven which the soul conceives when inflamed with the fire of divine love. Sublime is the excellence of mental prayer, great are its privileges; to mental prayer heaven is opened; to mental prayer heavenly secrets are manifested and the ear of God [is] ever attentive.” -Saint Peter of Alcantara