Mystical City of God is a book written in the 17th century by the Franciscan nun Venerable Mary of Jesus of Ágreda.
The book describes at length the various virtues, and how the reader should live in order to see them reflected in their own life, with the Virgin Mary as their model for sanctity.
[2] "My daughter, one of the misfortunes, which deprive souls of happiness, or at least diminish it, is that they content themselves with performing good works negligently or without fervor, as if they were engaged in things unimportant or merely accidental.
On account of this ignorance and meanness of heart few of them arrive at an intimate friendship of God, which they can attain only by fervent love.
For just as water is made to boil and foam by the fire, so the soul, by the sweet violence of the divine conflagration of love, is raised above itself and above all created things as well as above its own doings.