13 St. Francis published many works of piety, among which we may single out his two best known books, Philothea - An Introduction to the Devout Life and The Treatise on the Love of God . In the Introduction to the Devout Life St. Francis, after showing clearly how hardness of heart discourages one in the practice of virtue and is altogether foreign to genuine piety (he does not strip piety of that severity which is in harmony with the Christian manner of life) then sets himself expressly to prove that holiness is perfectly possible in every state and condition of secular life, and to show how each man can live in the world in such a manner as to save his own soul, provided only he keeps himself free from the spirit of the world.
Source: Rerum Omnium Perturbationem (Vatican.va)