VATICAN CITY — He stopped to pay his hotel bill a day after becoming pope. He wore simple black shoes and an ordinary wristwatch with a thick black band to his first Mass as pontiff. He rode in a minivan to dinner with the cardinals who elected him, affectionately telling them, “May God forgive you for what you’ve done.”

In an ancient institution where style often translates into substance, Francis, in his first 24 hours as pope, has dramatically shifted the tone of the papacy. Whereas Benedict XVI, the pope emeritus, was a theologian who favored red loafers, ermine-lined cloaks and erudite homilies, reviving papal fashions from centuries past, Francis, the former Cardinal Jorge Mario Bergoglio of Buenos Aires, appeared Thursday to be sending a message of radical humility.

“This choice indicates above all a style for the church: simplicity, poverty, rigor,” said the Rev. Antonio Spadaro, the editor of La Civiltà Cattolica, a Jesuit journal close to the Vatican Secretariat of State.