As the gift of counsel is given to us to direct our conduct by supplying for the imperfection of prudence, which would often remain hesitant, we need a superior gift to supply for the imperfection of faith.
This virtue attains the mysteries of the inner life of God only by the intermediary of abstract and multiple formulas which we should like to be able to sum up in a single one that would express more exactly what the living God is for us.
Here the gift of understanding comes to our assistance by a certain interior light that makes us penetrate the mysteries of salvation and anticipate all their grandeur.
Without this light, it happens often that we hear sermons, read spiritual books, and yet remain in ignorance of the deep meaning of these mysteries of life.
They remain like sacred formulas preserved in the memory, but their truth does not touch our soul; it is pale and lusterless, like a star lost in the depths of the heavens.
And because we are not sufficiently nourished with these divine truths, we are more or less seduced by the maxims of the world.
On the contrary, a simple soul prostrate before God, will understand the mysteries of the Incarnation, the redemption, the Eucharist, not to explain them, to discuss them, but to live by them.
It is the Holy Ghost who gives this penetrating and experimental knowledge of the truths of faith which enables the soul to glimpse the sublime beauty of Christ’s sermons.
It is He also who gives souls the profound understanding of their vocation and preserves them in this regard from every failure in judgment.
The gift of understanding cannot exist in a high degree without great purity of heart, of intention; it corresponds, according to St. Augustine, to the beatitude: “Blessed are the clean of heart: for they shall see God”.
Even here on earth they begin to glimpse Him in the words of Scripture, which at times are illumined for them as if underscored by a line of light.
St. Catherine of Siena and St. John of the Cross excel in this understanding of the mysteries of salvation that they may make us comprehend the plenitude of life contained in them.
R. Garrigou-Lagrange OP (1877-1964): The Three Ages of the Interior Life.