For example, pope Gregory XIII said that it was not murder to abort an embryo of less than 40 days. His successor, pope Sixtus V disagreed and made excommunication the penalty. But then came along pope Gregory XIV, who reverse this, making such 40 day abortions not punishable. Then in 1869 pope Pius IX decreed that the killing of an embryo was murder, decreeing excommunication as the penalty. Now either some of the popes were wrong when they spoke ex cathedra, which would mean that the pope is not infallible, or God just couldn't make up His mind.
Consistency is one test of truth.
A contradition simply cannot be true. Their word, then, should not be teated as equal with the word of God. The Catholic Church claims that many of its doctrines and practices rest upon apostolic tradition. In fact, the Church didn't even want the people to have the Bible in their language. They were so against the Bible being in the language of the people that act of translating the Bible was a crime punishable by death.