The Pope has no power of his own. He cannot (in his own name) excommunicate any person, revise the liturgy, or force anyone to do anything. He does possess the authority to teach, but this only in Christ's name; should any teaching of a given pope contradict that which Christ taught, whether directly or through the mouths of previous popes, he would cease to be pope. In other words, no pope can ever teach (for example) that abortion is not a sin or that one can attain eternal life by means other than Jesus Christ; were a given pope to do so, he would prove himself an antipope by definition.