This is the best argument I have ever heard for Catholicism (I heard it 20 years ago when I was a Mormon missionary.):
1. John 6 has consistently been interpreted to point to the mystery of the eucharist, and transubstantiation, even the earliest Martyrs agree on the doctrine of transubstantiation.
2. The Catholics are the overwhelming majority who have protected this holy mystery as part of the Church.
3. Some of the martyrs perished for their faith in this doctrine alone.
4. Dropping this mystery is terrible heresy, for it allows many come unto Christ that would not otherwise.
5. Heretic churches without this dogma condemn many people to purgatory.
Therefore the Catholic church is the catholic Church.