Once stationed in Poggibonsi, which was experiencing a period of strong economic prosperity, he abandoned his military career and married a young woman of the city, Buona de' Segni—traditionally called Buonadonna.

[2] His biographers state that, more than most merchants, he was so entirely and solely concerned with material success that he was generally reputed to be an avaricious man.

With his wife joining him in following a life inspired by their faith,[3] Modestini and Segni then had the option of separating and each entering a religious order.

This was an ancient and respectable way for husbands and wives to develop their spiritual aspirations, commonly practiced by married couples who felt a deep desire to follow God.

But Francis admonished them to persevere in their vocation, for he had in mind soon to give them a guide by which they could serve God perfectly even in the world, without entering into Religious life.

Francis then explained to them his plans for the establishment of an Order for lay people living in the spirit he preached; Luchesius and Buonadonna asked to be received into it at once.

The first simple rule of life, which St. Francis gave to the first tertiaries at that time, was supplanted in 1221 by one which Cardinal Ugolino prepared in legal wording.

Modestini asked her gently to please look in the pantry, for he trusted that Christ, who had multiplied loaves to bread to feed thousands, to bring the necessaries to the sick.

When Modestini lay very ill, and there was no hope for his recovery, his wife said to him, "Implore God, who gave us to each other as companions in life, to permit us also to die together."

Luchesius prayed as requested and Segni fell ill with a fever, from which she died even before her husband, after devoutly receiving the Holy Sacraments.