Mental prayer is the form of prayer taught to us by the saints that has practically been abandoned in the Church today—whether out of ignorance, laziness, or the action of the devil.
The Saint of Ávila, our true teacher of prayer, already said it: “The soul that abandons mental prayer does not need the devil to be damned; rather, it puts itself into hell with its own hands.”
Saint Alphonsus went so far as to teach that “it is morally impossible for one who neglects meditation to live without sin.” He even states that mental prayer is more effective than other private devotional practices because of its incompatibility with sin.
The reason for this, he said, is that those in mortal sin can persevere in these other practices, but no one can continue the practice of mental prayer while in a state of mortal sin. They will either repent or give up the practice of mental prayer
Source: Firmes en la fe