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. It is morally impossible for him who neglects meditation to live without sin.

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This is the chief fruit of mental prayer, to ask God for the graces which we need for perseverance and for eternal salvation; and chiefly for this reason it is that mental prayer is morally necessary for the soul, to enable it to preserve itself in the grace of God.

For if a person does not remember in the time of meditation to ask for the help necessary for perseverance, he will not do so at any other time; for without meditation he will not think of asking for it, and will not even think of the necessity for asking it.

On the other hand, he who makes his meditation every day will easily see the needs of his soul, its dangers, and the necessity of his prayer; and so he, will pray, and will obtain the graces which will enable him to persevere and save his soul.

(St Alphonsus of Liguori, Necessity and Power of Prayer, The Great Means of Salvation and Perfection)