well
1. History... the CC has been here for over 2000 years
2. Apostolic Succesion.. It has actual historical ties with the apostles and thus with Christ/God..
3. It went through the persecution described in Revelation when Rome went to war with the saints
4. The amount of works.. Tons of writings and great thinkers like St. Augustine and St. Aquinas.. The church has been through every possible question and answer that has been thrown at it.
5. Miracles.. The great happening at Holy Fatima and the great amount of miracles that happened in Catholic Church members(st. Padre Pio) . This shows where the holy spirit is dwelling and where the signes are comming from..
6. Universal.. Everyone in the world knows about being a Catholic.. But when have you thought of asking a monk in some tibetian tribe in the middle of nowhere to become a southern baptist?? In fact most of the newer protestant denominations like pentacoastalism are soley in the US, they are not universal like the CC is. Protestantism is more Regional while Catholicism is more universal..
7. Matthew 16