"For another thing, if the Pope is saying that capital punishment is always and intrinsically immoral, then he would be effectively saying – whether consciously or unconsciously – that previous popes, Fathers and Doctors of the Church, and even divinely inspired Scripture are in error. If this is what he is saying, then he would be attempting to “make known some new doctrine,” which the First Vatican Council expressly forbids a pope from doing. He would, contrary to the teaching of Pope Benedict XVI, be “proclaim[ing] his own ideas” rather than “bind[ing] himself and the Church to obedience to God’s Word.” He would be joining that very small company of popes who have flirted with doctrinal error. And he would be undermining the credibility of the entire Magisterium of the Church, including his own credibility. For if the Church has been that wrong for that long about something that serious, why should we trust anything else she teaches? And if all previous popes have been so badly mistaken about something so important, why should we think Pope Francis is right?"
How can Catholic "sacred tradition" be a valid source of divine authority when it is constantly changing?