Answer
Franz Brentano withdrew from the priesthood in 1873 because he was against the definition of papal infallibility. The definition of papal infallibility was made by the First Vatican Council in 1870, and it stated that the pope was infallible when he spoke ex cathedra, which means "from the chair". Brentano believed that the definition of papal infallibility was a mistake, and that it went against the teachings of the Bible.