by Thomas of Celano, Second Life of St. Francis
Francis Rebukes a brother who was sad and admonishes him and tells him how to behave.
St. Francis maintained that the safest remedy against the thousand snares and wiles of the enemy is spiritual joy. For he would say, “The devil rejoices most when he can snatch away spiritual joy from a servant of God. He carries dust so he can throw it into even the tiniest chinks of conscience and soil the candor of mind and purity of life. But, when spiritual joy fills the heart,” he said, “the serpent throws off his deadly poison in vain. The devils can not harm the servant of Christ when they see he is filled with holy joy. When, however, the soul is wretched, desolate, and filled with sorrow, it is easily overwhelmed by its sorrow or it may turn to vain enjoyments.”
The saint, therefore, made it a point to keep himself in joy of heart and to preserve the unction of the spirit and the oil of gladness. He avoided with the greatest care the miserable illness of dejection, so that if he felt it creeping over his mind even a little, he would quickly return to prayer.
Francis would say, “If the servant of God, as may happen, is disturbed in any way, he should rise immediately to pray and he should remain in the presence of the heavenly Father until he restores unto him the joy of salvation. For if he remains stupefied in sadness, the Babylonian stuff will increase, so that, unless it be at length driven out by tears, it will generate an abiding rust in the heart.”
From Thomas of Celano, Second Life, Omnibus.