After the fervor of the early mass conversions in Mexico had subsided, Franciscan missionaries came to realize that they needed a better understanding of indigenous peoples in order effectively to pursue their work. Sahagún's life changed dramatically in 1558 when the new provincial of New Spain, Fray Francisco de Toral, commissioned him to write in Nahuatl about topics he considered useful for the missionary project. The provincial wanted Sahagún to formalize his study of native language and culture, so that he could share it with others. The priest had a free hand to conduct his investigations. He conducted research for about twenty-five years, and spent the last fifteen or so editing, translating and copying. His field research activities can be grouped into an earlier period (1558–1561) and a later period (1561–1575).