So, if it's with the Baha', it's all fine & dandy; but if it's with the Catholics, no deal. Seems to be quite a double-standard.
You could post the sources of authority from God that the Pope used to make himself infallible. That would be the Word of God as Christ spoke to prove the infallibility of the Pope.
As a Baha'i I have those documents signed and sealed by the Messenger from God and always available to submit as proof.
The same Issue can be found in Islam.