Learning more about Aztec culture, Sahagún grew increasingly skeptical of the depth of the mass conversions in Mexico. He thought that many if not most of the conversions were superficial. He also became concerned about the tendency of his fellow Franciscan missionaries to misunderstand basic elements of traditional Aztec religious beliefs and cosmology. He became convinced that only by mastering native languages and worldviews could missionaries be effective in dealing with the Aztec people. He began informal studies of indigenous peoples, their beliefs, and religious practices.