In it he treats of the relationship between God and mankind, stressing first and foremost the love of God for human beings, and the plan he devised for saving them from the consequences of sin. Using many texts from the Old Testament Wisdom literature, he meditates on the extraordinary desire God has to love them and be loved by them. He, like St Paul and St John, sees the Saviour, Jesus Christ, as the embodiment of the Wisdom of God, and applies to Christ the title “Eternal and Incarnate Wisdom of God” — therefore the “Eternal Wisdom” in the title of the book is to be taken as Jesus Christ himself. In his reflections on the love of God for humans, St Louis Marie picks out the death of Christ on the Cross as the greatest manifestation of this love, to the point where he can say:
Wisdom is the Cross, and the Cross is Wisdom.
Making the assumption that all human beings desire happiness, and that “wisdom” (in a natural sense) is the way to achieve this, he discusses various forms of wisdom, and states that the only true wisdom is the Wisdom of God, incarnate in Jesus Christ himself. It then becomes the life-long quest of all Christians to seek to acquire this Wisdom, or, in other words, to seek to know and love Jesus Christ, the Eternal Wisdom of God. He proposes four principal means to achieve this:
An ardent desire to do so
Constant prayer for this grace
“Universal” mortification
A tender devotion to the Blessed Virgin.
He claims that this fourth means (devotion to the Blessed Virgin) is the most effective way to acquire and preserve Divine Wisdom, and it is this theme which he develops in the True Devotion to the Blessed Virgin and The Secret of Mary. The third means (universal mortification) is seen as a way of participating in the Cross of Christ, by accepting our own “crosses” and so becoming more aware of the love shown by the sufferings of Christ. He develops this theme more in A Letter to the Friends of the Cross.

Il trattato del Purgatorio