Either Catholicism is true or christianity is false. Why?
A. We have no tangible way to verify the correct books of the Bible (or that a Bible is even required) without a belief in an inspired early Church. To say that God inspired only the individuals that eventually led to the 66 protestant list of books over a period of 1500 years, logically leaves open the possibility that more books could be added or deleted, since the books themselves make no claim as to which are inspired and exactly how many are inspired.
B. If the Pope is not the legitimate ordained head over the earthly Church, then Christianity is subject to many contradicting voices. There is no final word that can be received with finality. This includes scripture and the Bible as well. The books of the bible themselves become a list governed by the majority of voters. If you get enough people to follow, promote, and market your List of inspired books, you can convince a lot of people to follow that list as the inspired truth.
But with Catholicism you have both a verifiable record of books that are inspired because the Church has the authority from Christ to bind and loose, and you have a single earthly head that can speak with one voice for the whole Church.
The Bible did not fall out of the sky and suddenly every Christian new which books to follow 1500 years after Christ. In Fact, the books that were considered inspired was changing not only many times over a period of 300+ years, but the list of inspired books (being read in the churches) was different from region to region. It wasn’t until after Constantine and the Edict of Milan in 313 that Christians could come together under one roof and start to determine which books should continue to be read and which ones should not. This wasn’t decided by the lay christians, this was decided by the Bishops. Individuals who were specifically ordained through the laying on of hands by the previous Bishops all the way back to the Apostles.
These same individuals believed in the Eucharist as the body and blood of Jesus and they were the ones that determined the list of books in the bible, with the help of the Holy Spirit who guides the Church in all truth.