10 What Francis wished to shine out, above all, in his Tertiaries, and which ought to be as their characteristic mark, is fraternal charity, most watchful guardian of peace and concord. Knowing that charity is the special commandment brought by Jesus Christ and the synthesis of the whole Christian law, St. Francis was careful to make of it the spiritual rule of his children; and he attained this result, that the Third Order rendered naturally the greatest service to the entire human family.
Source: Sacra Propediem (Vatican.va)