The Bible say’s that “there is one mediator between man and God, and that is the Christ Jesus.” Personally I have a problem with the Pope being a mediator, because the Bible tells me that there is only one.
You are right - there is only one Mediator who could die on the Cross for our sins, and that was Jesus Christ.
Jesus Christ appointed Simon Peter to be Chief Shepherd (that is, Pope) of His Church (John 21:15-19), with the expecation that he would appoint successors (Matthew 16:18-19), and that successors would be appointed until the end of time when Jesus returns and the Last Judgement takes place.
Pope Benedict XVI is 266th in
an unbroken line of succession that goes all the way back to St. Peter.
The Pope is not a “mediator” in the sense that Christ is a mediator, because the Pope does not die on the cross for our sins. Rather, the Pope is the key-holder and gatekeeper of the Church (see Isaiah 22:22, ref. Matthew 16:18-19).
The Scriptures also tell us in many places that we are required to obey lawfully appointed authorities. Indeed, we disobey our parents on pain of Mortal Sin (Exodus 20:12), and yet no one would say or think that their parents are always perfect, or that their parents are “mediators” with God.
Christ Himself established the Papacy, and Christ Himself directed St. Peter, our first Pope, to bind and loose, and promised him that whatever he commands for the discipline of the Church on earth is commanded in Heaven - if he states, for example, that for the purposes of discipline in the Church that we will abstain from meat on every Friday of the year, and that eating meat on Fridays is a sin, then God considers it a sin until the Pope himself unbinds that disciplinary law.
This works in exactly the same way that it works when a parent sets a disciplinary law in his or her home that the children must go to bed at 9:30 pm. If the children disobey this disciplinary law of their parents, they are committing a sin not only against their parents, but also against God, because it is God who set their parents in authority over them.