Mass is in itself more important than mental prayer. This is true, but without a life of prayer even the sacraments will have only a limited effect. Yes, they will give us grace, but that grace will remain unfruitful in part because the “good soil” it needs is missing. Why, for instance, are so many people who receive Communion frequently not more holy? The reason is that they do not have a life of prayer. The Blessed Eucharist does not bring all the fruits of inner healing and sanctification that it should, because it is not being received with an attitude of faith, love, adoration, and total receptivity—an attitude that can only be created by fidelity to mental prayer. The same is true of the other sacraments.
If someone, even someone very devout and committed, has not made a habit of mental prayer, something will always be lacking for the growth of his or her spiritual life. People like this will not find true inner peace but will always be subject to anxiety, and there will always be something too merely human in what they do: attachment to their own will, traces of vanity, self-seeking, ambition, narrow-mindedness, and so on. There can be no deep radical purification of the heart without the practice of mental prayer.
—from Time for God by Fr. Jacques Philippe, Chapter 1: Part 7