Because the Church tells you so. If you believe the Scripture you believe the Church. The difference is that a Catholic believes the Church consistently and a Protestant -- when his pastor lets him.
Is that the best you can do, because the "Church" tells you so? The "Church" at one time told me that I had to fast from the night before prior to receiving communion, then they said only one hour before, now it's you can if you want to. The "Church" used to tell me I had to wear a veil over my head attending Mass - even if it was only a hanky or kleenex - now you don't have to. The "Church" used to tell me that only good, obedient Roman Catholics could ever go to heaven, now it's really anyone who just tries to do their best, regardless of whether they even believe in God or not - except those nasty "Protestants" - they are hell-bound for sure. There is no consistency there to believe that is why I base my faith on what God's holy word says.