The third means of becoming a Saint is mental prayer.

by Saint Alphonsus Liguori

The third means of becoming a Saint is mental prayer. John Gerson writes: "That he who does not meditate on the eternal truths cannot, without a miracle, lead the life of a Christian. The reason is, because without mental prayer light fails us, and we walk in the dark. The truths of faith are not seen by the eyes of the body, but by the eyes of the mind, when we meditate; he that fails to meditate on them, fails to see them, and therefore walks in the dark; and being in the dark, he easily grows attached to sensible things, for the sake of which he then comes to despise the eternal." [De Med. cons. 7.] St. Teresa wrote as follows to the Bishop of Osma: "Although we seem to discover in ourselves no imperfections, yet, when God opens the eyes of the soul, which He is wont to do in prayer, then they plainly appear."  [Lettre 8.] And St. Bernard had before said, that he who does not meditate "does not abhor himself, merely because he does not know himself." [De Cons. l. 1, c. 2.] "Prayer," says the Saint, "regulates the affections, directs the actions,"  [Ibid. c. 7] keeps the affections of the soul in order, and directs all our actions to God; but without prayer the affections become attached to the earth, the actions conform themselves to the affections, and in this manner all runs into disorder.

We read of an awful example of this in the life of the Venerable Sister Mary Crucified of Sicily. Whilst this servant of God was praying, she heard a devil making a boast that he had succeeded in withdrawing a religious from the community-prayer; and she saw in spirit, that after this omission the devil tempted her to consent to a grievous sin, and that she was on the point of yielding. She forthwith accosted her, and by a timely admonition prevented her from falling. Abbe Diocles said; that whoever leaves off prayer "very shortly becomes either a beast or a devil." 
[Pall. Hist. laus. c. 98.]